A few things to add...
Conditions permitting, get 3-5 feet back from your bunker, don't crowd your bunker unless you have to (due ot angles, etc.)
Isolate your target -- using your bunker as well as others to the front and side of you. Ideally you only want only your target to be able to shoot back.
Do things in one motion, not a sequence of motions. If you're going out the top, you should simply just crouch, hop up and fall back down, shooting off a few shots for the time you are exposed. But like they already said, sides tend to be better since you can use angles to further limit who can fire at you.
You will learn, but it takes practice. If you get hit anywhere other than the mask or gun, you aren't using your cover well enough.