Take a drink and some food - you will get hungry and thirsty.
If in doubt, call yourself out - just because you didn't see where the shot came from doesn't mean someone else hasn't got you in their sights.
If you're caught in a bottleneck grow some balls and charge in - worse that will happen is you'll get shot (repeatedly) but if that concerns you then don't play airsoft. One of my pet peeves in game is people not pushing forward (especially in CQB).
Trash talking is never okay. If you do this you will be lit up the next time they see you. A bit of banter is fine but too much just ruins the fun.
Don't spam the trigger, especially at close range. CQB sites usually are semi auto only as being lit up at close range isn't fun. It only takes one shot to kill someone - three round bursts are what most people do to be sure of a kill.
Don't be afraid to talk to people or recommend tactics.
Give people a chance to call themselves out when hit.
Don't shout "call your hits!" - annoys most people and the marshals take a dim view on it at most sites. Chances are you'll get penalised if you shout it at other people. If you think someone is cheating then tell a marshal.
Most other stuff would mostly be common sense I'd hope - e.g. mags out in safe zone, always wear eye pro in game areas, etcetera!