Here are two exercises that cover the most used muscles in paintball, the longissimus and iliocostalis thoracis(back muscles) and the muscle group in the anterior femoral region(top of the upper part of the leg, specifically the sartorius muscle).
These two muscle groups control how quickly you can stand up and how long you can remain squatted.
Most workout routines don't cover these muscles because they are hardly used and are most difficult to target. So, you can do all the situps and leg presses you want, the growth of these muscles will be very slow.
A word of warning though, the three muscles are very long and thin, and there is some risk with these workouts, however, if you don't work them out and instead push them too hard playing paintball, it is even more risky. Above all, you don't want to "push them" as hard as you would any other muscle group there is no other muscles to support them and injuring them is really bad.
The first is the sartorius muscle, normally all this muscle does is rotate your foot outwards, so it isn't a very strong muscle, you need to change that: What you will need to do is squat down like you're in an "air chair," then spread your legs as far to the side as you can while still keeping your balance. Then shift your weight without moving your legs over top of each leg, then shift back to the other leg. You want to remain squatted the entire time and keep leaned forwards, no need to keep your back straight, basically remain in the same posture as you would if you were actually playing behind a bunker. The cool thing about this exercise is you can practice switching hands at the same time. Just move slow and steady, the sartorius is a very long and thin muscle and if you go too quickly or work it too much, it could do more harm than good. If you're doing it right, you should start to feel the pain on the inside of your leg just above your knee. As soon as your legs start to shake, stop for the day, if they aren't shaking, do some more reps until you think you've done enough(the shaking is very bad, don't let it get to that point). You're growing a small muscle, so it's going to take awhile, so don't try and push yourself super hard or expect to see a difference in 3 days.
The next is the back muscle, these are the ones that you don't want to use when lifting boxes, however, they are the ones that pick your chest and head up when popping up to play snake. Normally, these are bad to work out, because these are the two muscles that lead to back pain and if you push them too hard, that is what you will get.
However, you increase your risk if you just work these muscles out during normal play, so, it's good to work them out slowly in a controlled environment.
The exercise you want to do is kind of a reverse situp. Lay face down on your bed with your torso hanging off your bed and get someone or something to hold your feet down. Now, bring your chest up until you're back is straight, be careful not to lift too high, your spine isn't supposed to bend any further, and won't appreciate you forcing it further.
Another way is to do like you would in snake, squat on the ground sitting on your shins with your legs spread as wide as possible. Then pop up like you would if you were shooting over the snake segments, then go back down, over and over.
Again, take it slow, and don't push yourself too far. After you're done, lie on your back for a few minutes to let the muscles rest.
For even more info, read this thread: http://www.pbnation.com/showthread.php?t=3750737