Fire, move, fire, move, fire, move... If you stay in one place, they will move up on you and get the angles and you will be out. If YOU move on the other hand, you will be getting the angles on THEM, and taking THEM out.
Use trees and other stuff to keep your movements hidden. I often run to the side and when I get lined up with a tree that is in front of me. I sprint toward that tree, and then go to the next one. This allows you to get much closer without them seeing your movement.
Learn to snap shoot next to trees, as that skill is critical... ONE shot and hit your target, you want to be back behind the tree before your paintball even gets where it is going.
I often shoot out other players, and swing back out and fire again, before I see their hand up from the previous shot! Watching good first person video of games will help you "see" the field better, as most new players get focused on ONE movement or player while others get the angles on them!
check out my channel, it has TONS of full games on there, that will give you a good idea of what to expect, and how to move in the woods. Trails Of Doom's Awesome Woodsball Paintball FPS Channel HD Zoom - YouTube