Practice, practice and practice. I'll paraphrase Dave Tate and say that you should never feel comfortable while benching. A lot of people in the gym bench press because it's easy to be complacent - just flop down on the bench like a sofa and press. Do the opposite.
Hip flexor, quad and calf flexibility are big limitations as well. It's a good idea to stretch the lower body before your upper days in any case.
PVC is a good idea as well as 1 & 2 boards. I never used either though, stretching and practice were enough for me.