DO: show good gamesmanship - a simple 'gg/good game' at the end of a match goes a long way. Saying 'gf', 'gk' or 'gs' (good fight/kill/shot respectively) after a battle with another player also helps.
DON'T: gloat when you win. You may earn some payback later on in the match, in the form of a firebombing.
DO: talk with your team, especially in TC games. Using teamchat to mention contact with opposing units or where they may be going is helpful, while taking little time and effort on your part.
DON'T: complain about your team, especially if you've made no effort to work with them.
DO: switch sides in a match if one team is severely understrength. This tends to occur when a server is below half capacity and a player or two from one team disconnect.
DON'T: afk for extended periods of time (>2 minutes) during a match, especially if the server's full.