Here are some ideas:
The first thing you must do is get used to firing under fire. You can do it in practice again and again but remember that ultimately you are not playing in a vacuum and you must be used to how to respond when the paint starts flying everywhere including when rounds come flying against your bunker you will flinch so you must be prepared how to jump back and forth.
Another tip is to get as low to the ground as you can, I recommend prone if you aren't going anywhere quickly but if you plan on advancing or falling back stay in a crouch. The smaller the target the less liekly you are to get hit. By staying low to the ground, you also allow nature to take more of a toll on the paintball and it is harder to compensate.
Always know where you are going to shoot before you do. when you come out, you must be prepared to fire. Remember, you aren't going to hit diddly more than likely so be prepared to just put them behind cover, so a shot close by will work just about as well as a hit. Peek around the corner, size up your opponent andbring your gun up, set yourself into a tuck and then pop out, staying no longer than two or three shots. Snap back, assess their response and then respond accordingly.
Good luck...I will post more if I think of any.
For as I walk through the Valley Of The Shadow Of Death I fear no evil; For I am the Shadow of Death and evil must fear me.
"Get your facts first, and then you can distort them as much as you please." - Mark Twain