For quick games my only real advice is to keep moving and not sit and camp. If you allow multiple eliminations that goes double. If you have no reason to care about getting shot out then get moving and pulling off those crazy moves and stupid charges.
For longer games all I'm really good at is general sneakiness.
When moving wait for other noises to help cover your movements like traffic, a firefight, even a breeze remember everyone is wearing a mask so even a small noise can help obscure a few footsteps.
If you are trying to be sneaky don't make it too obvious and take really slow steps with frequent pauses. Try to be quiet but move steadily. People will pick up anything unusual (like footsteps that keep stopping every three seconds) but a steady walk will sound almost normal and that is good enough for paintball.
If you are trying to get the drop on someone top up your hopper to prevent rattling and any half full pods set on the ground and grab them later nothing will blow your cover like noisy pods shaking around.
Don't assume that everyone you see can see you and freak out and start shooting. Wait for a good shot and make it count. Sometimes somebody will start randomly shooting where they think you are. If you stay low and wait it out they will give up and I guarantee they will forget all about you. Paintballers have a short attention span so use it to your advantage.
If you are shooting from cover take two quick shots at a time. No matter how loud your marker is to you it sounds much quieter from the other end. Scattered shots are hard to place and it takes three or four to pin down where the shots are coming from. So take fewer but quicker shots to maximize your cover if you can make them count nobody will ever know where the hell your were. Which leads to...
After the game don't tell people where you were! Sure you are all buddies in the staging area but if you have a good spot to hide don't share it. The next time you get somebody lined up they will already be looking for you and you are no longer the King of Sneak instead you are just another guy drinking Gatorade in the dead box.