Paul's right, and to be a hair more specific keep the upper body over the back hip. This helps create separation by letting the hips get down the mound and ahead of the upper body. Otherwise you just rotate as one piece and that affects velocity and consistency because your timing is bad. I like a firm glove side that gets out of the way early, positive disconnection, you want to create separation with your scapula as well as the lower body. Pitching is all about timing and separation.
As for what to work on, I'm a big fan of focusing on correcting the lower half first. The lower half is your foundation, like building a house you've got to build the foundation before you start focusing on the next phases. All though I am a proponent of using driveline weighted baseballs to help shape movement patterns. Even just the drills that are a part of their program, pivot picks, rockers, roll-ins, walking windups, etc., can make a big difference because it breaks things down a bit and you can find more feel for how the upper half is supposed to work.