First Aid: CPR
Paintball can be very physically demanding, and as with any form of exertion, carries the risk of cardiac arrest. Two years ago, a man in Australia died of a heart attack shortly after having played paintball (I won't link to the source, as many media sources blame paintball impacts for the heart attack, which is idiotic in my opinion). The best tool we have for treating cardiac arrest is CPR, or CardioPulmonary Resuscitation, which aids in keeping cardiac arrest victims alive until paramedics (or the equivalent) arrive. I could easily go on for a long time about the details of CPR, but in the end everyone will benefit from something short, that demands their attention. On that note:
If you want to learn more about CPR and other life saving techniques, get in contact with a local branch of the Red Cross or other fine organizations such as St. John Ambulance. It's a small investment of time and money that could end up saving the lives of your friends or loved ones.