If you are under the influence of alcohol, illegal drugs, or prescription medicine, DO NOT PLAY PAINTBALL. Keep things safe and only play if you are fully coherent. 

Do not fire if you can't see what you are firing at. Many players are tempted to stick their guns out and fire without looking, but avoid this. Blind firing can lead to accidentally shooting players who are leaving the field, referees, or other things you shouldn't shoot. 

Close-range shots are considerably more painful than shots from a distance and it is customary to offer a surrender to any opposing player within twenty feet. Many fields require players to accept surrender if another player has come within twenty feet and has a shot on them. 

Keep paintball velocities under 300 (and generally under 280) feet per second (FPS). Gun speeds can be timed with a paintball chronograph (available to use at most pro shops), and should be carefully monitored. A paintball that travels at 280 FPS can cause small bruises, while a paintball that travels faster can cause significantly more damage including severe welts and broken skin. 

Whenever masks are not being worn, all guns should be blocked with a barrel plug or barrel sock. Safeties are good and should be used, but they often fail or are not properly used, and a physical protection from wayward paintballs is a must. Unless everyone has a mask on, do not take out your barrel plugs. 

Most problems can be avoided if you just use common sense. Don't shoot private property. Don't shoot out of a moving vehicle. Don't look down the barrel of a loaded gun. Don't shoot yourself, etc. If you ever wonder if something might be a good idea, it's not. 

This has already been said, but needs repeating: DO NOT REMOVE YOUR MASK WHILE A GAME IS STILL BEING PLAYED! Most severe injuries can be avoided if players simply keep their masks on.