Shoot everything that moves. This includes things that don't move. (Ex. Trees, squirrels, moose, people)
Take along lots of $$$. Them paint nuggets get expensive, especially when you are running a full/semi auto. Pumps are easier on paint, but don't give you as much covering firepower.
Pad up good. Reguardless of what some say, paintballs hurt like hell. Period. Its like getting shot with a giant BB-gun. Obiviously. Welts and redness are common unless you pad. (Don't go overboard. You gotta move freely)
Don't worry about the cost of paint. When you get out on the field for a round, forget how much ammo you got and concentrate on survival.
If you are in a position to shoot a team member from the other team, remember, he can shoot you too.
Staying low makes you harder to hit, but shots to your head is like getting hit with a junebug while driving down the road at 70MPH.
Leave the Rambo facepaint sh|t at home. Only newbies bring or wear it. True professionals play with a bright orange shirt, neon Hawaiian pants, and a cheazy gun. You see one of these guys, run like hell. Cause you will be lying on the ground sneaking up on someone, and this beer drinking bastard will put the muzzle of his gun in your ear before you can say "Aww schiet"