Honestly, what I've found that makes me 10x better at paintball to simply be to get in shape. One of the most important muscles is the one on the front of your legs that allows you to stand in an "air chair" for long periods of time, however you want strong leg muscles all around so that you can move easily without wearing out.
You could also consider some jogs and sprints, you may not notice it from all the adrenaline, but as you get tired you slow down and your reactions and movements suffer, so you want to be able to keep going as long as possible.
Now, onto your original question: the best thing to practice would be acquiring and shooting a target as quickly as possible. Compared to that, snap shooting is rather easy. With snap shooting you can keep the gun up and ready so you're pretty much focused on the target as soon as you come out. However, you want to work on getting a bead on a target as quickly as possible.
So, you want a wall that blocks off your view, then you want a target, you can set up some sort of apparatus that makes the target move, however you'll most likely have to resort to the other method.
Get a target that is flat and sturdy, probably something around 2 feet in diameter to account for the fact that paintball's suck at accuracy. Something that can survive repeated blows, then draw the standard target circles on it. After that, place a pole that will let you move the target around, so you can stick it in the ground to hold it up, but easily pull it up letting you move it around to change where the target is.
After that, you begin your practice.
At first, start off slow, you want to work on your muscle memory and technique, then slowly speed it up. So, when you first start, simply step outside and take your time, line up the shot so you hit it. Then come back in, turn around(not too fast, just enough so your body doesn't remember where it is) and point the gun at the ground, then come back out, aim at the target and shoot it. Remember: you want accuracy and precision THEN move onto speed and technique, not the other way around.
Why I recommend this exercise: I've been playing paintball for 10 years, over that time I became pretty decent at snap shooting with a semi. Then I moved onto a rock-n-cock stock class phantom. At that point, I discovered even with my experience shooting a paintball gun, I still had trouble hitting the target on the first try in the fraction of a second afforded to me. Once I tilted my gun down to cock it and reload a ball, I lost where the target was and I was slow to actually reaim the paintball and hit it on the first try.
If you do this exercise, your first paintball will be much more accurate and you'll spend less time correcting your stream and you'll be able to fire a stream dead on from the first paintball.
Always remember that you are absolutely unique. Just like everyone else.
98% of paintballers make up statistics 47% of the time. Post this in 16% of your signature if you're part of the 2% that doesn't make it up 83% of the time.