Trading: the underutilized skill
It seems remarkable to me that I find many good players (some friends of mine included, who I yell at from time to time) both neglecting to trade and not understanding how trade works. I'll write a bit about the way it works, then open the floor for input on when the rest of you feel it is most appropriate to trade.
1) trade carts/cogs cost 100W/50G each. Oftentimes, when you've researched Caravan you can nearly make that amount back in a single trip.
2) trade functions best when the markets are as far as possible apart (subject to the constraint that you don't want your carts running through unprotected territory, or around too many obstacles of terrain/buildings.) I've tested this to the maximum diagonal distance across a giant map, and a single (Spanish) trade cart can yield an amazing 190+ gold in a single trip. Most times you'll have to settle for 120-130 though, the amount that you can get along a border of a large map.
3) trade carts train BUTT slow. You need to make multiple markets in order to quickly crank out the number needed for effective trade. Often I'll make 2-4 extra markets in an ideal location (against a map wall) and delete my first one once it's researched the trade upgrades.
4) contrary to popular belief, the resource gathering rate curve for trade carts has a steady positive slope out to the maximum distance possible to travel in the game, i.e. using TEAMWORK, establish your trading system in the longest safe distance between you and your teammate's base.
5) the resource gather rate for trade at yield distances of 100 or greater is comparable to what you can get from gold miners WITH an upgrade
6) there is no limit to the number of trade carts you can use (unlike in AOE, where a dock could only support a certain number at a time.)
My personal opinion is that trade should be established as early as possible, considering the military threat presented by an opponent. If no threat is forthcoming, having 8-10 carts created or in q just before hitting Imperial is not unreasonable. From there on, I like to build up to 30 or 40 total, sometimes more depending on the gold requirements of the civ I am playing. There is *nothing* more frustrating for an opponent than to build up a 100 or so halberdiers (the bulk of his army) and to see a Frankish player who has been furiously trading continue to send wave after wave of Paladins at him. Of course it is not efficient to send ONLY paladins at such an opponent, but the sheer fact that you CAN match his halberdiers 1:1 when you've been trading effectively will win the game for you. And the great part about it is that unless he's done very effective scouting deep into your base, he'll have no idea that you've been collecting an insane amount of gold for the past hour, and that you're more than ready to fight a protracted battle with gold-heavy troops.
Before I get a bunch of flames related to "they should never let you build up that long," or "I'd just flush you," or the like, stop and consider that much fun can be had in games where you follow something other than the 15 minute recipe that's been posted a million times here. If Ensemble had intended that players *ONLY* optimize the hell out of the first 15 minutes of the game, and flush/counterflush/Castle rush every game, then they certainly wouldn't have included the concept of trading into the game--there are plenty of standing resources on the board to support those *boring* kind of games.
*Personal record in a 2-hour game is 108,000 in trade gold. Man can you make a lot of Paladins with that.