Yeah, Gamo and Body pretty much said it all.

You need technique firstly and secondly your whole entire body needs to be conditioned to deliver a punch... upper, lower and core.

I swear by hill sprints for developing explosiveness in the legs... you should always do the positive part of the rep (the contraction) quickly and the negative part (eccentric) slowly... gonna give you long muscles and explosive contractions.

You need to have an efficient Anaerobic energy system to you need to work intervals of near to or on your maximum effort... burpees, punch out drills etc.

Scrap will swear by stretching, he's the man to listen to about that, your muscles are basically elastic, very very important part of muscle contractions and body mechanics as a whole.

Trick is not to do Anaerobic training and plyometrics too much, got to recover... it can play hell with your Central Nervous system and Lactic acid...