fightingforever, do you throw your hook palm in or palm down/elbow up? if you truly "turn the hook over" you can get much more weight behind it, rather than the slap you often see with amateurish "palm in" left hooks.

also, you can do full body weight training meaning no isolated movements using machines... do something that emulates the actual motion. for example, for a left body shot, you might toss a heavy slammable medicine ball against a concrete block wall with as much force as possible, using the exact same motion as the punch itself. . that is a plyo, but a plyo that mimicks the real thing to a T.