One thing I find that most vets hate (I'm still a newbie myself) is "sniping." i.e., someone who hangs out in a hide, doesn't move around at all, and just picks people off.
While I don't understand this mentality (it IS a war game scenario) just be aware thats the way many of the vets feel.
Understand the difference between cover and concealment. Cover won't let a paintball get you. Concealment MAY.
Paintball ain't like shooting a firearm. Paintball is won by the VOLUME of paint thrown at teh opponent. Aimed fire is pointless. Just blast away with a constant flow of paint. You'll get lucky and get some easy kills.
NEVER take your mask off in the field of play.
NEVER EVER EVER wipe paint off DURING the game. The vets HATE that, and besides, its uncool. It'll likely get you several paint balls to the groin, at close range.