I had an instructor recommend that when you're at the range (where such action is allowed, after warning others, of course,) have someone randomly do something *VERY* distracting right next to you (jab your off-arm, for example,) yelling at you constantly, when you try to shoot. Jump up and down and jog in place very fast immediately before going to the line. In a life-and-death situation, you're not going to be calm, you're going to be working on adrenaline. Practice like that once in a while, and see how bad your aim gets.
Sure, shoot out the kneecap, that's fine. *AFTER* you have put two in center mass.