Don't laugh..this really does work--
Practice throwing harder. That has been the biggest tool i have used with my pitchers. When they ask me how to improve their velocity I pull them aside and let them throw to get warmed up and thentell them the biggest secret of all- I say "OK, now I want you to throw harder. I usually have them throw in bursts of three building upon each one a little harder each time. The results have been amazing using this simple approach.
I always tell them that if they never practice throwing harder, they will never be able to throw harder- it is a skill that must be learned through practice.
What I have found is that when my pitcher actually tries to throw harder, his mechanics improve naturally on their own- their body moves quicker, they get more separation through their torso to upper body. It is amazing!