It's not how hard you hit it; it's how WELL you hit it.

-Clint Eastwood, Million Dollar Baby




But seriously, I use a total gym for sport-specific moves...wrist moves for my hockey shot, certain arm moves for my throwing arm and hitting in baseball and jab-like moves and pseudo-one-armed push-ups for boxing. Plus it has an attachment for shoulder moves. Beyond that I hit the heavy bag a lot. I'm just a newb though, so I'm not yet a "boxer" but I do hit pretty hard if I do say so myself. And whenever I'm learning something new, I do it very slowly and methdically to develop a natural feeling and motion, like learning how to kick with your other foot or switch-hit in baseball. Proper technique leads to power...or so I've been told. Good luck.