The main thing is to not be a pain. Courtesy toward your allies and enemies alike is the golden rule of MP. Sure, you can talk smack before, during, and after the game (provided you arent just being a jerk)--but conduct yourself in a courteous manner befitting a knight.

At some point you have to admit defeat. 3 v0 peasants aren't going to chain rout 2.5 full armies....so why try to hide them in the forest for that last "sneak attack" in the 1/2,000,000,000 chance that they will win the day?

Anyway, good topic Az. I've been thinking of compiling a concise list of globally accepted multiplayer etiquette rules to be posted in the Newb area.