Here are a few tricks and tips I have picked up. Some of which I found out myself, and some of which were taught to me (just, passin' them along!).

• The AT List is your best friend - especially when preparing to begin charging. Use it to your advantage to make sure your target doesn't move from the panel or towards one of your characters.
• When you move your characters, try to move them to a spot where an enemy that must fight up close (knights, monks without Wave Fist or Earth Slash, etc.) cannot hit you unless they move directly in front of you. This works especially well with your Thieves and Samurais who have high innate evade. After the enemy unit falls for this trick, you may move to the side, or if your movement is 4 or higher, move behind the enemy for an easy hit.
• Bringing in a Chemist or someone with Item and Throw Item set into a battle is usually a great thing. Not only that, but Chemists can equip the 8 panel range guns, which can keep them out of danger. Any job with Item and Throw Item on usually works great (especially with close range fighters or those that have problems defending themselves)
• Be aware that the enemy will usually go after the character that can harm the enemy party the most. By job, the enemy likes to go after the magic users first, then healers, then ranged attackers and then close range fighters. Unless you have one character that is overly more powerful than any other characters or you have one that is so weak that the enemy can kill it with one or two shots, this is usually the pattern they follow. In Chapter 2 at the execution site, I brought in a knight equipped with Gafgarion's equipment from the beginning of the Chapter (which raised the knight's HP to 200). Gafgarion and the characters right near him immediately bum-rushed the knight and ignored the fact my Summoner, Lancer, and Agrias were attacking Gaf from behind (the knight was in Squad 2, while the other three were in Squad 1).
• In order to protect Crystals and keeping the enemy from receiving them, have the Lancer Jump on the crystal to keep the enemy away from the Crystal (after all, who wants to voluntarily gain 100 or so HP's only to lose 250 or more?). This works especially well when one of your characters needs the crystal bad and the enemy is closer to the crystal than you are.
• Another great ability to possess is Concentrate, an archer ability. With this equipped for battles with highly protected knights, it completely nullifies all their evade. This is especially great for Archers, because then they only have to stay in one place and can shoot down anyone they want. This is also good for the Elmdor battle in Chapter 4, as Elmdor not only has Genji equipment, but also possesses Blade Grasp (combined with his high brave, it's not a pretty site for your close range fighters).
• If any of your characters have equipped rare or powerful items, consider giving them the Maintenance ability. Particularly, this ability is useful for the battle with Meliadoul or anyone else with Mighty Sword techniques, as they will try to break your most powerful equipment.
• Slow as they are, Calculators are one of the best jobs in the game. They can reach far across the battlefield with any of the spells they have learned from the jobs of Wizard, Priest, Time Mage, and Oracle. They are most effective as Attackers, and not as much Healers, but Boosters in a way (those rare equations that hit all members of your party and none of the enemy are great for spells like Haste, Protect, Wall, etc.). Just remember that with power like this, the enemy is sure to make eliminating the Calculator a preference. Not only that, but Calculators have the worst Speed Change out of all the jobs. Strangely enough, if you gain 5 levels as a Calculator, your innate speed increases by 1 (the fastest speed growth in any job). Unless you're looking for an ultra fast character, learning all the Math Skills and maybe Move-Get JP/Exp and Gained Exp Up are your safest bets, then changing to a faster job with more magic attack like Wizard or Summoner. Then just make Math Skill your secondary skill and you're golden.
• Archers are the most annoying characters to fight against - particularly when they start at a height, because they can virtually cover the entire battlefield without even moving. Archers tend to either attack your weakest or strongest character, or use their strongest Charge on one of your guys with low CT. Not only that, but if the Archer has a Long Bow, it doesn't matter what's in between the Archer and the Target - the Archer will hit the target, with only a few exceptions. Any archer that is on the same height as the target or one or two panels has one glitch that can prevent the arrow from reaching the target. That glitch is if you move one of your characters in between the archer and the target. Using your own judgement of predicting the path of the arrow, chances are the arrow will hit that character and not the target. This is great especially if the Charge from the Archer will kill the target.
• You may have already noticed this, but anytime an enemy is charging something it is very vulnerable to attack. Unless the unit possesses Blade Grasp, they're evade percentage drops to zero from all angles and any physical damage is increased by 50%. This is the best time to consider bum-rushing those annoying Summoners, Time Mages, and anyone that needs to charge something - especially if one of your units is the intended target. This is also helpful to remember about your guys - the enemy is a sucker to hit any of your guys if they are charging. That is why it is good to only use spells that won't activate until so many units get AT. If your Wizard wants to charge a spell that won't activate until 4 enemy units get AT, there's a good chance your Wizard won't live to see that spell activate. If you must do that, try to move that character to a spot where he/she will be out of danger (behind some of your guys, up on a height, away from Melee combat, etc.)
• An Archer that is equipped with Ignore Height is your best friend. Archers are best from a height, and if they can immediately jump from height 4 to height 12, you'd get quite a good range on those that are still on height 4. Ignore Height is also good for your spellcasters, as they are safe from melee combat if they are on a height 4 or more panels from those that are close range fighters.
• How about that ninth level of Deep Dungeon? In that level, there's a 1 in 4 chance that you will run into 6 ninjas and 2 Samurais. Go ahead and try to run into this group - only prepare your characters with the Reaction ability Catch. Imagine 6 Ninjas chucking Stars, Swords, Knives, Katanas, and the like at you while you are catching them and keeping them. If you are on a higher level, say 95, then they start chucking rare items at you like Excaliburs, Masamunes, Chaos Blades, etc. This is definitely the easiest way to get those rare items that you must explore for in the Deep Dungeon.
• Speaking of Deep Dungeon, be sure to save each time before you go to a new floor. Each Floor as 4 items and there is a Brave percentage chance that you will find a Phoenix Down instead of the item that is usually there. Your best bet is to take one of your generic characters with Brave 50 or lower, equip them with Move-Find Item, then when you get into the battle in deep dungeon, use the Oracle Spell Foxbird on them to lower their brave 30 points. Although at the end of battle you will have permanently lost 5 brave, you will only have a 10-20% chance of finding Phoenix Down instead of the regular Item. This is why you want to save before each battle - some of the items you find in Deep Dungeon are rare and you only have one chance to pick them up (either the Item or a Phoenix Down). If you want to raise that character's brave back up, just use Ramza's Guts abilities or the Mediators abilities.
• Want a mighty character other than Orlandu? Equip Ramza with the following abilities and the Job Knight: Jump, Weapon Guard, Two Swords, and any Movement ability of your choice (preferably something like Jump+2 or Move+3). For his two swords, give him two Knight Swords. This creates an unstoppable Melee and Long-Range fighter. Not only that, but maybe even a Shelled and Hasted character to boot. This also works great for anyone else with good innate attack power.
• The ability to invite enemies into your party is something to rave about, if used wisely. It's most effective on generic human enemies with higher level equipment than you have. If somewhere on your attack team exists an archer with a Windslash Bow (strongest purchasable bow), inviting an enemy archer with an Ultimus bow is great. This works on anybody. Just invite the enemy in, strip them of their equipment, dismiss them, and give the equipment to someone of your regular attack team.
• Another way to use this to your advantage is to even the odds. If you're up against 8 units, inviting 2 of those enemies as guests in this battle would tip the odds in your favor. You don't have to take them into the party, but they will help that battle to completion.
• It can be frustrating to build up calculators, especially due to their lack of speed. Here is an easy way to do just that. First, be sure the unit has access to the sing or dance action abilities. Learn the HP or MP dance/song (preferably the HP). Then when the calculator gets AT, activate that song or dance. The dance/song usually will activate about 3 or 4 times before next AT, and each time the song/dance activates the calculator gets EXP and JP (providing some healing/damage was done). Overtime the calculator will have amassed enough JP for mastering, and can change to a wizard, with Math Skill secondary.
• The lancer's Jump is the only ability that needs charging that you can't see when it will occur on the AT. The charge speed is unit speed * 2. You should only jump characters of equal or lesser speed with current CT below 50. With slower characters to jump, a few CT points can be sacrificed. Never jump hasted characters, unless their CT is horribly low (25 or lower, as general rule of thumb).
• Thieves are also difficult jobs to build up, as their Steal action is rarely useful in random battles. You can always use the sing/dance trick like with the calculators, but a better idea is to learn some Monk Skills (preferably all of them) and Martial Arts. Change to a thief and equip Punch Art and Martial Arts. Not only will you receive a decent fighter, but Steal is still there when you need it.
• Oracles are better than once reputed, especially if the unit to change has fighting job history. With their two-panel range sticks, they can be quite powerful. I changed Ramza into an Oracle (and Ramza had already mastered Knight and Monk), and with an Octagon Rod he was taking 160 or so. Plus, the abilities that an Oracle has can hamper the enemies efforts greatly (Petrify, Spell Absorb, Paralyze).
• The Samurai support ability Two Hands is great for any job that, after equipping, has a free hand. It's also great for Lancers, as damage is increased greatly (for a job that already does great damage). It also increases melee power for non-fighting focused jobs (Summoner, Oracle, Calculator, Thief, etc.)
• Ninja support ability Two Swords goes great for any job - especially fighting jobs. Magic focused jobs get a fighting boost, while Knights and the like are immensely powerful.
• Although I did do this my very first time going through the game, DO NOT just build up 5 characters in your attack team. It's much more useful to have multiple characters in your party, who specialize in one or two particular jobs. If you just build up 5 guys, then you won't have certain jobs up to snuff when you need them. If you have 13 guys, each with one or two different jobs mastered, they'll be there when you need them.
• Whenever storyline characters join your party (controllable), have them master their unique job before moving them to other common jobs (Agrias and Orlandu can use Counter and other related reaction abilities quite effectively). This being they are the only characters that have those abilities (and the majority of them are quite useful). The only storyline characters you really don't need are Rafa & Malak, because their abilities aren't accurate (and even if they were, it's rare you get 3 or 4 enemies in an effect range of 2).
• Speaking of mastering jobs, it is a good idea to take one character of your attack team and focus that character on the Wizard Job. Overtime that character will master the job, and believe me, the mastered Wizard will definitely be a force to be reckoned with. Even if you change to a fighting job, give that character Black Magic and MagicAttackUP. This can create a powerful character if combined with the right job.
• The above goes for a Summoner as well. Save JP for the Wizard's MagicAttackUP, then master the Summoner Job. Again, even in a fighting job, with Summon Magic and MagicAttackUP, the character will still be very powerful with the Summon Magic.