Just keep throwing. Be careful not to throw out your arm, but just go out and throw till you speed drops off, your muscles will build for that kind of motion just like any other work out.
its kinda like jumping. Wanna dunk? add some resistance and keep jumping.
@kibaxcheza has a good idea. Also, if you can find a way to add a little weight to a baseball, the resistance idea should work in building up and training the right muscles.
How old are you? A pitcher will always want throw throw harder but if you’re too young and try to develop you muscles too fast you’ll end up throwing your arm out in high school. There’s no secret, cure-all, fix-all trick. Just do it the old fashioned way, keep working hard and it’ll come. The proper grip helps also; you don’t want to be completely palming the ball.
@frdelrosario, that’s a good point that I didn’t mention. I’d say 75% of a pitcher’s power comes from his lower half. If you remember Mark Prior, he had the biggest legs I’ve ever seen.