Steve's book, TuffCuff is a solid explanation of how to become fit to pitch. In layman's terms, which is very handy.
By doing some strength training your body will improve it's athleticism and ability to activate motor units in a short amount of time. Thus it should allow you to better your velocity. Although it very's person to person, expecially with the frame you already have.
Mechanics can do a lot. I added a few miles per hour this fall in a matter of days after posting a video of my throwing and people pointing out tweaks I should make.
There are plenty of people who know mechanics around here so I encourage you to get some footage if possible.
Out of curiousity have you been done growing for a while, or still possibly have some room to grow?