I agree with the other posts, but not entirely. I believe that you have to work from the ground up. As you may already know, 80% of velocity is created by the seperation of the hips and shoulders as the front foot stabilizes. this means you want your hips facing home and your chest facing third base when your front foot hits the ground.
I have included a clip of Roy oswalt, who does exactly this at 24 seconds in the video.
The first thing you have to work on is utilizing your back leg. Watch how he loads his glute up and his weight is on his heel. he then explosively rotates off the rubber, and when he lands, his drive leg is fully extended and his shoulders are closed.
Every hard thrower in the mlb gets into this position. Look at video of chapman for somebody who is near perfect.
SO along with keeping your lead leg closed as long as possible so you dont leak stored energy, you have to ride your back leg down the mound and the powerfully rotate it.
let me know if you have nay questions buddy.