Form speed and strenth as Milash said is exactly right.. You need to train your entire body for your calf to your neck.. Balance out your whole body.. Then work on having the perfect form and balance.. Much much power can be gained just from having the weight distributed correctly during the punch, and using the right form.. Practice explosive moves like clap pushups and alternative things like woodchopping, or get and old tractor tyre from a tyre yard and belt it with a sledge hammer over and over..
Everything will contribute to the punching power,, but you won't get it without good form..