In response to what steve ellis said, I, too, am also doing those kind of exercises, and so far, my mechanics are becoming sharper in the process and my velocity is increasing due to the fact that my pitching coach is making sure I throw with authority and accuracy every pitch. He is actually going to tape me today to show certain movements that I do that effect my accuracy and velocity, as well as arm angle ( my velocity may just be increasing because i'm learning how to use my trunks and back leg more, although, I have noticed more explosiveness while using thera bands and doing specific medicine ball work).
I've begun to stray away from heavy weightlifting and do more med. ball work, band work, and just going over my windup in front of a mirror. I do the weightlifting to gain weight since I have a very high metabolism. Although, if you're looking to gain strength, you could start doing squats, just make sure you spend some time on getting the form right. Forward, backward lunges, deadlift. Having the biggest arms in the world may not necessarily mean that you can throw 90 plus. With that in mind, just focus on strengthening up your back muscles, legs, core muscles, shoulder (rotator cup), forearm, triceps, etc... Also, do plenty of sprints, and occasionally, long distance runs.
Some good exercises to do with the medicine ball could be, stretch out like you would throw, but hold the med. ball real loose, and just relax your arms so that your holding the ball about where your waist is, or a little bit higher then that, then use your trunks to quickly rotate to apply force to the ball, this should enable the ball to come out and your hands should swing out. The goal here is to explode with your hips, and not use your arms too much, maybe just a little, you're not throwing the ball, just letting your hips guide the ball out. I hope that makes sense, sort of hard to explain.
Some other ones could be, doing a soccer pass with the medicine ball against the wall, goal here, explosiveness, about 12 reps, umm, sitting against the wall so that your legs are bent and your using your upper back to rest on the wall. Hold the med. ball next to your head, then follow through, do this on both sides for about a 30 seconds to a minute, goal, explosiveness. If you like to know about any more just feel free to ask, sry for the long message.