Step 1: Profile: Playing Tight
Profile is how much of you there is for the enemy to shoot at. You want a small profile so that you are less likely to get hit.
Playing tight is staying as close to your cover as possible to minimize profile.
Keep your elbows in.
Keep the muzzle of the gun behind the corner so you can snap back behind should the need arise.
Shoot lefty around the left of cover. Shoot righty around the right of cover.
Remember to keep your legs and butt in. Its easy to forget that these are not in cover.
Distinguish between cover and concealment:cover blocks incoming fire, concealment hides you but may not block bbs.
If you remember to do all this you will be less likely to be hit. In the pictures I highlighted what would be available to shoot at. The first one is bad form.
Quick tip: by placing the elbow of your trigger arm (that is, the one whose finger is on the trigger) on your rib cage and placing the stock on the outside of your shoulder, you geometrically guarantee that you have the smallest profile possible, so long as your muzzle is as close to the edge of cover as possible.
EDIT: Having had a few years more time to practice this, and having acquired a better camera, I will ideally be slowly rewriting this over the next few months.