If you don't already have them get goggles that are olive drab green. People really look for black so olive drab can help.
The human eye sees movement first, then shape, then color. Try to look around just using your eyes and move your head as little as possible.
Don't get tunnel vision. You should look ALL THE WAY AROUND YOU at least every 10 seconds. Even more if you know there's opfor nearby.
Don't just drain a hopper at everything that moves. I know this is how alot of people play now days but it's not skill. I've always thought that the better you are the less paint you will go through in a game.
Learn how to make paint grenades and use them. After a while you will get really good at using them. I can drop a grenade pretty much anywhere I want one. Usually the first person I hit with one is the guy that thinks grenades are useless. lol.