Firstly get a heavy bag and put in some serious time, on the first day - hit every shot your hardest(using good form), train till you drop then train a little more, if you feel your gonna die then go at it a little more. Dont worry about speed and technical moves, just punch- hard!!! The next day you will feel hurt in some places, these are the places that should be developed. I got this tip from someone else and was shocked- no triceps, chest, biceps shoulders pain, what hurt most was my lats and middle back, neck, abs, and my forearms/wrists.These are vital to power punching. A good way to get wrist strengh is to get some tips from armwrestling websites.