The last person you'd expect to deliver does just that
Examples:
Kirk: during a team bowling competition, one of Kirk's teammates kept getting just 1's in her throws... but then ended up getting 3 consecutive strikes at the end to win.
Family Matters: Urkel hitting a big shot in a hoops game.
Full House: in a hockey game one of the regulars scores the game winner from a penalty.
Recess: Gus, the skinny kid with the glasses, waxes everyone at dodgeball.
Rocket Power: Sam, the fat kid with glasses, waxes everybody at kickball. (courtesy Don Del Grande)
The Little Lulu Show: with the score tied 9-9 in a soccer game (seriously), the person who scored all 9 goals for Lulu's team throws the ball long. Iggy trying to get the ball runs into A goalpost... the ball bounces off Iggy's head and into the goal.
South Park: Pip, the kid with the British accent, waxes the Chinese at dodgeball.
Hey Arnold!: Titular character, after having been benched for the entirety of several games, scores game-winning points in hoops game with a free throw and a set play, after the team (rather, their coach) had insisted on relying entirely on one player (the coach's son, as it were)... in fact, Arnold had been benched for passing to his friend Gerald instead. That same coach later coaches Arnold's bowling team, and it comes down to classmate Eugene and a wide split. Eugene falls on his face when he throws the ball... and yet somehow makes the spare for the win.
Exceptions:
Fresh Prince of Bel Air: a hoops game ends with Carlton Banks, after grabbing the ball from Will Smith, with their team down by one... airballing the final shot.
The Simpsons: Bart is the 2nd string QB (thanks to some unexpected, and unwanted, "special treatment" from his dad, the coach) and after performing badly in one game, asks to replace first string QB Nelson when there's a warrant against him during the final... he's "replaced" into the police car, leaving Nelson to score the game winner.
The Weekenders: Tino practices horseshoes all, uh, weekend... still gets beaten by the resident school jock Laird anyway. Also, in another ep, Carver races him, and after a pep talk from fellow friend Lor about the fact he isn't racing in his lucky shoes, and a bunch of hustling, we have a photo finsh... and Laird still ends up beating Carver anyway.
Honorable mentions:
Rocket Power: The aforementioned Sam is also the goalkeeper on his friends' roller hockey team. In one crucial game (where the winner would get to play with a team of NHL all-stars), the team replaced him with resident bully Lars, expecting his larger size to be advantageous... he ended up running up a 4 goal deficit after 1 1/2 periods. However, after putting Sam in, the team didn't concede another goal. Despite this, the team lost that match when a game-tying goal at the final horn from Lars' younger brother Twister was disallowed.
Doug: Title character goes hitless in baseball game between the team he's on and the school team... until he switches to left-hand hitting and finally gets a hit (see below, however, for why this is just an honorable mention).

The last person you'd expect to miss does just that
Examples:
Clifford: score's tied 4-4 in a soccer game, Vaz passes the ball to Jetta, who takes a shot with full confidence... it goes off the bar, and there goes the final whistle.
Doug: After Doug's teammate Patty (who formed her team after being rejected by the school team) had hit on every at-bat and right after Doug's hit (see above), her potential game-winning lob was caught by the school bully Roger.
Happy Days: Ritchie misses a free throw he needed to hit to tie state championship hoops game... wonder if anyone got the "have a lifesaver" joke at the end.

Offspring of former rivals compete against each other... the offspring main character usually gets revenge over their senior's defeat
Examples:
Hey Arnold!: Grandpa Phil was once beaten in model boat race by Rex Higgins, Arnold's now up against Rex Higgins the 3rd in the same race.
Rocket Power: Ray Rocket was once beaten by Chester McGill in a sports competition, Otto Rocket's now up against Theodore McGill in the same competition.
(It should be noted that in both cases, the senior rival turns out to have cheated, with Rex poking holes into Phil's boat and Chester taking a shortcut during a downhill ski race.)
Exceptions:
Step by Step: After the mother on the show mentions to her teen daughter that she was "robbed" of winning the head cheerleader position when she was in high school, it turns out the person who's just won the head cheerleader position is the daughter of the person who had won the head cheerleader position from mom. (You following me here?... However, the cliche usually calls for the 2 former rivals meeting each other early on to set things up like in the examples, as opposed to after all is said and done, as is the case here.)

Girls beat boys in competition
Examples:
Little Men: Girl wins skiing competition, embarassing the boy who finished 2nd.
Punky Brewster: Punky wants to race radio-controlled model cars, but the boys who race them, and her father, don't want her to; of course, she ends up winning the big race. (courtesy Don Del Grande)
James at 15/16: James becomes the coach of a girls' basketball team - which challenges the team he's on after his team wins the school's intramural league. (courtesy Don Del Grande)
Rocket Power: Otto's older sister Reggie scores game-winning points in rugby game, and this was after it seemed no one wanted to let her play.
Exceptions:
Doug: the aforementioned baseball game, though it is a close game, and Patty's told that she's probably good enough for the school team.
Honorable mention:
Rocket Power: Reggie crowned first ever female winner in the aforementioned competition between Otto and Theodore (see below for more detailed circumstances, however). (Given that Reggie is a "girl power" character, you'd think they'd use this cliche more often. :-/ )

A third competitor wins in a competition with 2 major rivals
Examples:
Mike, Lu and Og: In a run around the island that exchange student Mike is organizing, princess Lu tries to cheat her way to winning by jumping on Mike, who immediately runs into a tree (Lu's hands were over Mike's eyes), leaving Lu's turtle Lancelot (the only remaining competitor since Lu either jumped on them or they pooped out) as the winner.
Detention: With genius twins Lemonjella and Orangejella (voiced by the Mowry twins, no less) disqualified from a spelling bee after one of them butted into the other's turn, Gug (the only remaining competitor) is your winner.
The Weird Al Show: Fred Huggins wins TV Host of the Year after Weird Al and his most significant competitor, Uncle Bobby, tried to sabotage each other's shows.
SpongeBob SquarePants: Squidward's "pedigree" snail falls in love with SpongeBob's pet snail after the latter crashed into a wall during a snail race, leaving Patrick's "snail", actually a rock, to win.
Rocket Power: The two significant rivals in a sandcastle building contest are Reggie and Lars... but your winner is a young kid named Mackenzie Benders (she's a recurring character, and it's one of her most notable moments in the series).
Bachelor Father: the two most talented girls in the school compete in a talent show, but the winner, "based on originality", is a girl who does impersonations of celebrities reading "Mary Had a Little Lamb", an act the title character's (John Forsythe, of Dynasty fame) daughter was going to do before she decided that it was more humiliating than talented. (Probably the earliest examples of this cliche, as Bachelor Father dates from the 50s.) (courtesy Don Del Grande)
Small Wonder: Vicki and Harriet beaten in talent contest by daughter of shopping mall owners who were judging the event. (courtesy James Vipond)
Dexter's Lab: Dexter's big sis Dee Dee and Mandark's little sister compete in ballet. In the end, a third girl wins the applause of the crowd at a ballet concert. (courtesy Katrover)
Tiny Toon Adventures: Plucky and Babs are in a contest to see who can appear in the school yearbook most often (with Plucky cheating mightily). The winner: "The kid in the orange hat" who appears in the background of every picture and who turns out to be Buster in disguise. (courtesy Viceroy)
Brady Bunch: Marcia and a girl Greg wants to date both try out for a cheerleading spot. There are three girls trying out, and the committee creates a three-way tie (one vote for Marcia, one vote for the girlfriend and one vote for a third girl). As president of the committee, Greg has to cast the deciding vote. He votes for the third girl, who performed the best. (courtesy Deseree Graham)
Totally Spies: Mandy and Sam are bewildered that they have both been beaten in a youth contest... by a 50-year old teacher.
Teacher's Pet: Leonard Helperman and Scott Leadready (or should that be Spot?) run against each other for class president... and the winner (by one vote): the other candidate, their wacky classmate Ian.
Honorable mentions:
Rocket Power: In the competition between Otto and Theodore (mentioned above in the "offspring" and "girls beat boys" cliches), Reggie wins in a tie with Otto (though only after Theodore declared that he cheated during the race).
House of Mouse / Mickey Mouse Works: In a little informal contest, Goofy wins a ticket to a rollercoaster over an over-competitive (well, against each other) Mickey and Donald, by painting more of said rollercoaster than the other two.
Little House on the Prarie: A variation on the cliche involving an election for school president: Mary, Nellie, and Elmer are running. Elmer is a boy who isn't popular and is frequently picked on at school. Mary ends up dropping out of the election when she realizes that Elmer would be the best one for the position. Elmer does end up winning by one vote. (courtesy Deseree Graham)

First hurdle failure by "know-it-alls"
Examples:
The Honeymooners: Ralph's trained hard for a gameshow (a ripoff of the $64,000 Question) in the category of music. He misses on the first question, about who wrote Swannee River (he thought it was a friend of his, Ed Norton, since during the training, he always played the intro to this song).
Hey Arnold!: Harold is a fat kid and an avid eater. In one ep, there's a neighborhood eating contest, however, he doesn't do as well as anyone would have expected... he's out during the first stage. (For the record, your winner is the title character of the show, in rather unusual circumstances, with Arnold's sole remaining rival suddenly falling face first into a huge bowl of ice cream and Arnold simply eating the cherry on top just to fulfill a technicality.)
Full House: Stephanie's trained hard for a spelling bee... and ends up getting eliminated on her first word. She's not so pleased herself because she got the word "mnemonic", pronounced without the "m" at the beginning.

Regular(s) defeated on game show... because the regular(s) getting wealthy would upset the show's status quo
Examples:
The Honeymooners: Ralph on $64,000 Question ripoff, see above.
Dinosaurs: There was an ep where the main characters were on a game show, and their defeat was partially due to all of the questions being about TV (they weren't exactly TV addicts).
The Simpsons: In one of the Christmas eps, one short scene showed Marge ending up in the red on Jeopardy!.
Animaniacs: Brain stumped on Jeopardy! ripoff in Pinky and the Brain short (ironically, the answer to the question that stumped him was Ralph from The Honeymooners).
The Odd Couple: Felix and Oscar beaten on Password. (courtesy James Vipond)
227: Mary and Sandra beaten on Wheel of Fortune. (courtesy James Vipond)
Golden Girls: The "Grab That Dough" ep. (courtesy James Vipond)
Exceptions:
Rugrats: Tommy's mother wins a statuary on a game show fronted by Alex Trebek (in a reversal of the situation on The Simpsons).
Spin City: One character won the top prize on Who Wants to Be a Millionaire? some time before it was actually won.
Flintstones: Barney wins houseboat on a ripoff of (the old) The Price Is Right. To be fair, this is the whole plot setup. (courtesy James Vipond)
Small Wonder: Lawsons beat Brindles on a show similar to Family Double Dare. (courtesy James Vipond)

Friends or family make lousy opponents
Examples:
The Simpsons: Bart calls a tie against his neighbor's kid Todd Flanders in a mini golf tournament, and also does so against his sister Lisa in a hockey game.
The Weekenders: The gang has been split into two teams of 2 for a "pudding ball" tournament with other kids... they end up being the final 2 teams in the tournament, but somehow both teams end up getting disqualified.

Guy saves some other guy, other guy then offers to do stuff for him... usually ends with the other guy ending up saving the first guy to even things up
Examples:
Brady Bunch: involving two of the brothers and getting trapped in a meat locker.
Rugrats: involving Angelica and Chuckie.
Small Wonder: After Vicki sacrifices herself to save Harriet from drowning in the Lawsons' pool, Harriet wants to do favors for Vicki. (courtesy James Vipond)
Exceptions:
Hey Arnold!: Arnold saves Sid from a falling sign... but this story doesn't end with Sid saving Arnold... instead, Sid complains about how Arnold is taking advantage of him, prompting Arnold to get a certificate for pancakes at the store the sign belonged to for Sid so that they're even.
Gomer Pyle, USMC: Carter pretended to get knocked out by gas so that Gomer could "save" him... but ended up having to save Gomer again.

The "no buts" cliche: One guy does something objectionable (at least the other person thinks so) and the other person continuously berates the first guy, not giving him a word in... and usually the first guy has something to say that would clear things up
Examples:
8 Simple Rules: One of the teen daughters gets detained for shoplifting and John Ritter continuously berates her for it... in reality, the friend she was shopping with did it.
I Am Weasel: Baboon is late for a motorcycle test, and Weasel berates him for this... turns out he forgot the brakes to the motorcycle.
House of Mouse / Mickey Mouse Works: Donald tries to build a brick wall at girlfriend Daisy's house, and she keeps berating him for (for example) dancing with her neighbor (the neighbor literally dragged Donald into it).
All Grown Up! (the Rugrats spinoff): Dil appears on a talk show, "What's Your Tragedy?", about Tommy abandoning him upon winning an award in filmmaking, prompting booing from the audience when Tommy shows up to tell his side (basically, that it's not the case at all).
Full House: D.J. was trying to take a beer can away from two boys at a school dance when her uncle Jesse catches her in the hallway and wrongfully accuses her, until the two boys who were drinking confess to Jesse and he apologizes to D.J. (courtesy Sascolts)

Playing hookey gives character worse time than if they stayed in school (dates from the days of The Little Rascals)
Examples:
Hey Arnold!: School has an unexpected carnival on the day Arnold and Gerald decide to play hookey. (In other cliches, principal Wartz talks about how he would have been a pro hockey player except that he lost his toe in a thresher accident... but that cliche is primarily a movie cliche.)
Rocket Power: Replace "unexpected carnival" with "assembly involving extreme sports"... which would make it a disappointment for Otto and some other kid, who play hookey here, since they're extreme sports lovers (the show centers around sports, actually).
The Little Lulu Show: Lulu and Tubby play hookey to get out of a geography test to watch a film premiere... and end up discovering the film premiere was sold out, and that the test was replaced by a trampoline factory field trip.

A character is about to do something for the first time... and something precludes it altogether
Examples:
Full House: Michelle's first time in a horse-jumping competition, precluded by Bob Saget arguing with her would-be competitior's mom, leading to Michelle pulling out along with the competitor.
The Wild Thornberrys: Debbie's first meeting with her email date, precluded by her younger sis Eliza having to get an operation for appendicitis, leading to father Nigel (voiced by Tim Curry, no less) having to borrow Deb's intended mode of transport, the "Mini Comm" (a motorbike with a sidecar), to take her to hospital. (Despite the fact that she'd been waiting for this meeting for over a year and that she wouldn't be able to meet him again for another 17 months, she handles this circumstance very well, actually... mother Marianne says so at the end.)
South Park: the school's first win (in decades) over a rival school, precluded by QB Stan searching for his missing (and gay) dog, leading to him missing most of the game, leading to the rival school thrashing South Park.
Rugrats (the pilot for All Grown Up!): Tommy's first concert, precluded by his being grounded, due to him taking his dad's medallion for his (actually, Angelica's) personal purposes (read: wearing it to said concert in order to be picked from the audience to sing with the performing popstar, or in Angelica's case, just to show off to her best friend)... his friends eventually bailed him out though.
Hey Arnold!: School bully Helga Pataki's bid for her first trophy (just to be like her more-successful older sis Olga), precluded twice by her own conscience: the first time, it was her catching her dad trying to bribe Arnold to throw a spelling bee (she then, quite clearly, deliberately missed a word on her turn when it came down to just her and Arnold, thus letting him win), the 2nd, it was the fact that her best friend, school genius Phoebe, had let her win the qualifier for the city academic competition (she then pulled out just before the competition was to start, in spite of all her preparation, and put Phoebe in instead... guess who won the competition?).
The Charlie Brown and Snoopy Show: Charlie forced to forfeit his baseball team's first win (in quite some time) over a gambling scandal (read: Rerun betting Snoopy a nickel that the team would win). Taken, like most of the other sketches on the show, from the comic strip.

Something that's been "secret" from the viewers for a long time about to be revealed, but then isn't
Examples:
Cheers: Diane throws pie at Sam at a dinner... it ends up hitting Norm's wife Vera in the face.
Hey Arnold!: In two episodes, somebody is reading Arnold's name, but they can't read the last name (in fact, in one episode "Crush On Teacher", even Helga wonders what his last name is). Also notable is when Grandma Gertie burped loud while Grandpa Phil was reading an account of the wedding of Arnold's parents.
The Simpsons: On numerous occasions, about where Springfield is. ("Hey, Casey Jones, where's this train headed?")
The Fairly OddParents: We hear a truck whoosh by instead when Timmy's parents (as kids... Timmy has travelled back to when they were young) are introducing themselves to Timmy ("But you can just call me Dad/Mom").
Saturday Night Live: They had a sketch where they were going to reveal the sex of Pat, Julia Sweeney's mysteriously androgenous character. Pat was about to walk into a restroom when they interrupted the sketch for a special news report. (courtesy Paul Hippen)

A kid is away for a few days and a friend or relative is supposed to care for their pet and through their carelessness the pet dies... sometimes, the friend or relative goes out to the pet shop and buys a replacement hoping the kid won't notice, but they always do (courtesy James A. Jones)
Examples:
Home Improvement: Except that instead of replacing the pet fish, the kid who's taking care of it just weighs it down with lead balls and tells the owner that he's training it to do some tricks (like balancing on its tail).
Rocko's Modern Life: with a turtle
Rugrats: with a bug named Melville

Getting caught doing something nasty because of someone coming back earlier than expected
Examples:
Inside Schwartz: In one of the eps that never aired in the US, guest star Alison Munn told Schwartz that she was hired by MTV to cover the Super Bowl. Now, Schwartz's agent had just phoned him and told him he was being considered for the position, so after she left for a dinner, Schwartz gave him a call telling him about it, adding that Munn wasn't exactly talented (he'd been with him for quite some time trying to refine her sportscaster techniques, even though he was told to disencourage her from taking up this profession). Munn heard that particular comment, however, since she had just come back in to get her purse, and guess what: She got mad at Schwartz for not telling her that earlier.
Rocket Power: Ray decides to take a dip into his neighbor Merv's high-tech pool since he heard that he and his wife were gone for the day (taking anger management lessons)... soon after he takes a dip however, they catch him in the pool (they came back only because Merv needed to check the system backup).

A character dates someone who looks and acts exactly like one of the other regular characters (but they don't notice the resemblance) (courtesy Paul Hippen)
Examples:
Love & War: Annie Potts dated a guy just like Jay Thomas.

A couple breaks up and one or both of the characters and their friends go to a strip joint to cheer the sad person up (courtesy Paul Hippen)
Examples:
Suddenly Susan
Friends

Girls want to do what boys do, so boys do what girls do (courtesy Don Del Grande)
The Brady Bunch: Marcia wants to join a group of boys-only scouts, so Peter joins the "sunflower girls".
Diff'rent Strokes: Kimberly wants to join the school's lettermen's club (or, as she wants it renamed, the "letterpersons club"), so ("Whatcha talkin' 'bout") Willis runs for homecoming queen. Willis wins, but the principal then decides that if boys can be homecoming queens, girls have to be allowed into the lettermen's club.
The Partridge Family: one of Laurie's friends wants to join the boys' basketball team, so one of Keith's friends runs for homecoming queen, and Laurie then runs against him; Laurie wins, but gives him the title, and the other girl ends up on the team. (Little-known fact: in California, up through about 1980, there were no "boys' teams", just girls' teams and "student" teams (open to boys and girls), but then somebody realized that if boys could be banned from taking spots away from girls, girls could be banned from taking spots away from boys if there was a girls' team in the same sport, so while girls can still compete in, for example, football and wrestling, they can't play boys' basketball if the school has a girls' basketball team.)

Falling in love with someone... who's in love with someone else (or other combinations)
Examples:
All Grown Up!: Nicole Boscarelli, a girl at Tommy Pickles' and Chuckie Finster's (and the rest of the gang's) school, falls in love with Tommy... problem: Chuckie has already fallen in love with her (under unusual circumstances: see below in the "be yourself" section). In fact, this cliche is the tone of the ep in question, with the "wrong" people in love with the "wrong" people.
As Told By Ginger: In consecutive eps (production-wise), Ginger's best friend Dodie falls in love with a boy, who happens to actually be in love with Ginger's other best friend Macie; and then she falls in love with an exchange student, who happens to fall in love with Ginger (he even kisses her during the school play Ginger had signed Dodie up for just so that Dodie could get closer to him... however, Ginger never got around to explaining to him how Dodie felt about him).
Hey Arnold!: Arnold "likes" Lila, who falls in love with Arnold's cousin Arnie...who, it turns out, falls in love with Helga. There's also the time where Gerald falls for an older girl... who only has eyes for his (uninterested) older brother Jamie O.

Work sabotaged, but saved, well, sort of
Examples:
Totally Spies: Alex, Clover and Sam suspect that Mandy was deliberately sabotaging their photograph when she supposedly accidentally opened the darkroom door while they were developing it... however, much to Mandy's surprise (and for that matter, the other girls'), the resulting overexposed photo wins a photo contest they'd submitted it in, over Mandy's professional-looking photos... apparently, the judges decided that it was "fresher" than those photos.
Doug: Roger isn't good at making banana pudding in a cooking contest, but thinks he's at least ruled Doug and Patty out of the competition after accidentally spilling some on what was supposed to be their pizza (for that matter, so do Doug and Patty)... much to their surprise though, the resulting "banana pie" is your winner.
Detention: Emmitt Roswell's science project has been ruined by a classmate's pig... but the aforementioned Gug helps him rebuild his project into one on the Big Bang, which gets an A. All's well that ends well, as Shakespeare would say (the main plot was on a school play based on Hamlet... Emmitt suspected that Gug ruined his project and decided to get him for it).

Abandoning talent for something that doesn't seem as important
Examples:
Hey Arnold!: Mr. Hyunh (person who lives in apartment building Grandpa Phil runs) announces he's quitting country music circuit to live his simple life as a restaurant worker. "Forgive me for not taking your brass ring," sings Randy Travis (the voice of Hyunh, singing, in that story). Also, there's an incident involving Stinky turning down a contract to do soda commercials without any apparent justification (he confides with Arnold that he doesn't want to be exploited for his negative qualities, which is what the batch of commercials he's just filmed is doing).
Hangin' With Mr. Cooper: Raven-Symone abandons chess for her family. IIRC, her screen mom wasn't keen on having Raven spend her days training for the world championships.

A character giving up something s/he wants (bad) for a best friend (or relative)
Examples:
The Weekenders: Carver gives up ticket to skating fest he'd wanted to see to buy Lor educational computer game to help her study for Civil War test. Proves worth the time, since Lor ends up getting a B+ on it. Tino: "It's tearing you apart, isn't it?" Also, Carver and Lor reluctant to give up the cash they'd saved up to go to a concert so that Tish could buy Tino and his visiting dad (which in itself is a cliche, see below) two blimp tickets.
The Wild Thornberrys: Debbie gives up first meeting with email date for Eliza (see above). (Also of note: Eliza gives up her power to talk to animals to save Debbie from being thrown off cliff by Rupert Everett, by telling him about it, in the film based on the series.)
Hey Arnold!: Helga gives up spot in city academic competition she'd trained so long for, to the more deserving Phoebe (see above). She also gives the designer snowboots she got for Christmas (and that she had been wanting for so long, I might add) away to someone at a government office so that he'll search for a person Arnold has been trying to track down (more on that below).
Honorable mentions:
Rugrats (All Grown Up! pilot): Angelica gives up concert ticket to Susie to help Tommy return his dad's medallion (and right when he needs it as well). Angelica still gets to go to the concert, however, since her grandfather has an extra ticket (which had originally belonged to grandpa's new wife).
Rocket Power: Otto gives up chance to win race against Theodore (see above) to help a broken-kneed Twister finish a snowboarding race. Otto still wins (although shared with his big sis), however, after Theodore's disqualifaction for cheating.
Also, see "Abandoning talent for family", above.

Fight leads to passion (courtesy the webmaster of this Scrubs fansite)
Examples:
Cheers: the aforemenmtioned Sam and Diane.
Frasier: Niles and Daphne.

The self-directed lecture, where the details gradually shift to apply more to the lecturer (courtesy the webmaster of this Scrubs fansite)
Examples:
Scrubs: Elliot couldn't cope with the idea of dating male nurse Paul and Carla was having trouble accepting Turk's marriage proposal. Carla lectured to Elliot about men and commitment, ending with: "as scary as it is to consider letting yourself be truly vulnerable with another human being, what's even scarier is that deep down inside you know you picked this man...and if you run away from him now, you'll be running away from being the kind of person you always wanted to be."

Important speech (and the like) messed by hypnosis et al (courtesy Bill Steele)
Examples:
That's So Raven: Victor accidentally hypnotized by Cory... and this is just before he's supposed to be sharing his recipes live on local TV.

Well-known saying gets put down
Examples:
Sabrina, the Animated Series: Sabrina's trying to get a gift for Salem and all she could find was a cheap picture frame... when her best friend tells her that "It's the thought that counts", she says, "That's what cheapskates say."
The Weekenders: The gang has told Tino, about a sports competition he'll be competing in, "It's not whether you win or lose, it's how you play the game"... Tino says in reply, "That's what losers say." In another ep, Tino says "Two wrongs don't make a right"... Carver, having been publically humilated at the local beach by someone, argues that "One wrong doesn't make a right either, but two wrongs at least makes me feel happier", before slipping a forged note under that someone's door to dupe the person into wearing scuba gear at the local mall.

The Body (and Voice) Switch (courtesy Viceroy)
Examples:
Teen Titans
Jimmy Neutron: with the two smartest kids in school, the titular character and his classmate Cindy Vortex
Kim Possible: with Ron and Kim
Lloyd in Space: with Lloyd and baby sis Francine (though to that show's credit, they didn't switch voices)
Power Rangers

A character who has gotten "A's" their whole lives gets a "B" and flips out (usually accompanied with a "What's wrong with getting a 'B'?" spiel)
Examples:
Power Rangers: Billy
Simpsons: Lisa (there was Kamp Krusty, and the time where she played hockey to avoid getting her first ever "F", in PE)
Hey Arnold!: Olga (but this is only because her younger sis deliberately changed one of her "A" grades to a "B+", and it's a "B+" and not the usual "B")
Meego: Titular character takes the phrase "My life is over" quite literally after hearing someone say it upon getting a "B" on a test
Exception:
The Weekenders: The gang is trying to find the group's intellect Tish a new identity for her after getting a B on a test, but she doesn't see what the fuss is all about: "It's no big deal, you get B's all the time" (this ep hasn't been shown in the US yet)
Honorable mention:
The Weekenders: Tish receives her report card, and it's "Straight A's as usual" (OK, it's not usual when you consider the cliche)... but she is disappointed (to say the least) to read a negative comment about her perfectionism: what moral was this ep trying to tell us anyway?

The "just be yourself" story... character changes self radically in order to impress another character... most of the time, ends up impressing the other character most when the character is just himself (or herself)... usually used in "preachy" cartoons (courtesy Paul Evans)
Examples:
The Weekenders: The whole gang did this in one ep for a video shoot, but there have been quite a few other eps with just one character doing this (mostly to, yes, impress another classmate... in fact, 3 stories alone have been devoted to Lor, her crush Thompson, and this cliche).
Pepper Ann: Pepper's mother is going to be interviewed for TV, and has rented some furniture for this... after a 30 minute story on stuff reserved for special occasions (e.g., the "velvet room" in Milo's house and the lawn in front of the James sisters' house), mother decides to revert back to their original furniture instead upon the interviewer telling her that she was being interviewed for being down-to-earth.
All Grown Up!: Chuckie Finster's idea of impressing the aforementioned Nicole: creating a new alter ego, an exotically-dressed Latvian, leading to hilarious circumstances when Nicole would like Chuckie to meet "Chongo".
Hey Arnold!: At least three examples: 1) Mr. Simmons wins some award, and there will be a documentary made at his class, so what does he do? He gets his kids to rehearse the "perfect day". 2) Helga does a little makeover on herself... during a brief talk in fashion-conscious Rhonda's bathroom however, Phoebe tells her, "I liked the old Helga more, at least she was honest." 3) Sid "borrows" Arnold's room to study because he doesn't want a classmate to think that he's plain and boring.

Christmas specials where a character learns that the meaning of Christmas is family (or other intangible values)
Examples:
Too many to count actually, but here are a handful I've seen myself.
Sabrina, the Teenage Witch: she restores Christmas to the mortal world in one special by visiting St. Nick himself and learning that Christmas is really about family
The Weekenders: Lor's Grandma tells about her worst holiday ever to the gang (they're not having a good one either this year), ending by saying that "Christmas is about being with who you care for"... and that "you gotta stay away from wild turkeys" ;-)
Hey Arnold!: In lieu of trying to find Mr. Hyunh a birthday present, Arnold decides to track down Hyunh's daughter. He ends up having to do some Christmas shopping anyhow, as the person at the government office willing to help wants them to do some for him
Exceptions:
Dexter's Lab: a short with Dexter chasing Santa ends with Santa telling Dexter that Christmas is really about presents

A visit from the divorced dad
Examples:
As Told By Ginger: Ginger's dad Jonas visits at Christmas after Ginger's previous attempt to get him to listen to her reading a poem
Braceface: Sharon's dad is a drummer. Meanwhile, she's scheming to keep her mom from dating a teacher
Rocket Power: Sam
The Weekenders: Tino's dad is just as concerned about germs as he is

The perfect replacement - an annoying and/or lazy regular character is temporarily replaced by a "perfect" (polite, hardworking, etc.) substitute (courtesy Paul Hippen)
Examples:
Cheers: Carla was replaced this way.
Northern Exposure: Dr. Fleischman was replaced this way.

The attractive woman who just can't get a date (courtesy Paul Hippen)
Examples:
The Nanny: C.C.
According To Jim: Kimberly Williams
Hangin' With Mr. Cooper: Holly Robinson

The invisible character, talked about but never shown ... maybe the writers feel that the character has been talked about so much that actually having an actor play the character would be a disappointment (courtesy Paul Hippen)
Examples:
Cheers: Norm's wife Vera (see above)
Frasier: Niles' wife Maris
(Note that Frasier is a spinoff of Cheers.)
Golden Girls: Dorothy's cross-dressing brother, Phil, is talked about, but never seen, including the episode where he dies of a heartattack and the girls attend his funeral. (courtesy Megan Devine)
Designing Women: Suzanne's maid from San Salvador, Consuela Valverde, hassles her constantly and is consistantly mentioned, but never makes an appearance. (courtesy Megan Devine)
The Weekenders: The gang has often mentioned this disaster-prone girl named Chloe Montez, but since we hardly ever see (and thus get to know) their classmates (this is, after all, a show about the weekend), we're not even sure if she's ever made an appearance yet, but if she has, she certainly hasn't been identified as such. Examples of her wacky life: getting "eaten" by a dinosaur robot and getting lost in a bathroom dressed as a contact lens.
Cybill: Mary-Anne's husband, Dr. Dick, a very convenient source of money and funny dialogue for both her and Cybill (courtesy Celticsweetgrass)
Columbo: His wife (Courtesy Laurent-Paul Robert)
Charlie's Angels: Of course, who can forget the heard-over-the-speakerbox, seen-from-the-back-of-his-chair Charles Townsend? ;) (Courtesy Laurent-Paul Robert)
Exception:
Just Shoot Me: Jack Gallo's attractive but dumb wife Ally was originally "invisible" but (to the producer's credit) they finally had an actress play her (it was not disappointing).
Honorable mention:
December Bride: the neighbor Pete Porter (played by Harry Morgan) was always complaining about his wife Gladys. We never got to see Gladys. When the show went off the air they had a spin-off called "Pete and Gladys" which starred Harry Morgan with Cara Williams playing Gladys. Show wasn't much more than a low-grade "I Love Lucy." (courtesy Walt)

The temp employee from hell (courtesy Paul Hippen)
Examples:
Just Shoot Me: Cheri Oteri
Newsradio: French Stewart

The boss who's sort of spacey and out of it (courtesy Paul Hippen)
Examples:
Just Shoot Me: the aforementioned Jack Gallo
The Faculty: the principal
Newsradio: station owner Jimmy James

Major character's older sibling who has at best a supporting role... encountered primarily in animated shows
Examples:
Rugrats: Susie's older sibs Edwin, Buster and Alisa... Susie herself doesn't get much role in the series though (on the other hand, that's likely to change with the All Grown Up! spinoff)
Rocket Power: Twister's older brother Lars
The Wild Thornberrys: Eliza's older teen sister Debbie
Doug: Doug's new-age loving older sister Judy
Teamo Supremo: (Captain) Crandall's older sister Jean ("I need to know this stuff if I want to be a famous (fill in occupation here) someday")
Hey Arnold!: the aforementioned Helga's older sister Olga and Gerald's older brother Jamie O
As Told By Ginger: Darren's older sister Will and Brandon's older brother Stuart
(And note that in many cases, the older sib is a much different character from the major character, for example, Eliza likes adventuring with animals while Debbie would rather be back in the US doing teen stuff, Twister is best friends with Otto while Lars likes to bully Otto and his gang around, etc.)

Tough chick with soft spot she's not willing to admit
Examples:
Project Geeker: the title character's protector Lady MacBeth (as for how tough she is: she's got a cool-looking biomechanical arm that shoots projectiles... enough said)
Hey Arnold!: the aforementioned Helga (many eps' plots surround her attempts to keep a lid on her crush for Arnold... one of the show's noteworthy eps has her talking about this crush to the school psychiatrist, voiced by Kathy Baker)
Jimmy Neutron: the aforementioned Cindy Vortex (she's tough on Jimmy because she used to be the smartest kid in class before he moved in)

A cartoon duo that consists of one stupid character and one smart character (courtesy Dark Spider)
Examples:
Ren and Stimpy
Animaniacs: Rita and Runt, and Pinky and the Brain
The Fairly OddParents: Cosmo and Wanda
CatDog
The Angry Beavers: Norbert and Daggett

The slim, spunky and agile action girl who uses an array of kicks on the bad guys
Examples:
Kim Possible
Buzz Lightyear: Mira Nova (plus, she's got an arm mounted laser and the ability to walk through walls... and did I mention royal blood, too?)
Road Rovers: Colleen (see below if you think I'm suddenly making up things)