We must start with the obvious. Never take your mask off or lift it up while on the fields. There can always be someone after a game that forgets to keep their fingers off the trigger, has not put a barrel plug back into their barrel, and/or not placed the gun safety on. Referees will be vigilant about this rule. Keep your mask on!
When the game starts, run! Field position is key. Get up field fast when the game horn sounds. But don't get beyond your lines. Make sure you have team mates willing to do the same otherwise you may be too far beyond your lines allowing the other team to maneuver around you for a clear shot at you.
Don't shoot! That's right, don't shoot until you are ready to give away your position! If you've moved up field fast, you may have a great position. Let your opponent move within range and then take a few quick shots at them looking for a break of paint. Bounces don't count, so if paint doesn't break you'll need to keep shooting.
Don't call yourself out! Well, sort of. If you are hit, don't assume you are out. Paintballs do bounce sometimes and if you call yourself out, you're out. So what do you do when you are not sure?
You call "Paintcheck!" This will draw a referee to you. The referee will inspect you and when the ref determines if you are hit, they will call you out or announce, "Players clean!" if you are clean, then you can proceed back to playing.