Hm, I always assumed it was a matter of practice, though having stronger and flexible muscles probably doesn't hurt.
I googled "improve punching power" and found something decent over at
http://stickgrappler.tripod.com/fb/fbpower.html It's some basic advice, no special workouts or anything.
From the article: "One last point for you, is that you have to develop your kinesthetic perception if you really want to be able to hit with power. There is a correct sequence of muscular contractions for every punch, every kick, which happens very quickly in rapid succession. In addition to contractions, there is a correct series of RELEASES (very important) -- i.e. when a certain muscle in the chain has done its job, and now needs to let go before it starts to hinder your function."