OK, Firstly JKD is pretty close to the truth on this one, a punch from a standing position derives its power from a chain like reaction through each joint in the body, each adding a little bit more to the final resultant force. Secondly whilst most weight training does only give you strength there is a little confusion arrising on this issue. Any isolation exercise should be avoided for starters, a punch is not an isolated movement. Secondly, compound explosive exercise such as the clean and jerk are good for teaching the body to explode through a movement, however, plyometric weight training does improve the explosive capacity of the muscle and therefore results in greater POWER and therefore applicable strength to the movement. As well as good mechanics and porper synergistic excersises to complement this, remember that nothing is better for improving a punch than punching and being aware of what it feels like to hit things and also to be hit. I rigged up a pressure monitor in the garage and started wacking the damn thing with different techniques, from different positions, isolating certain bits of me, taking bits out, hitting blind fold, hitting before training and after training and I learned what a punch that takes a head clean off feels like to throw and when I am most capable of applying it.
Finally, remember that any antagonistic tension will act like a break on the agonist, so a big bicep is just a big break with a punch, so learn to relax the bicep when hitting. The mid point between proximal and distal hand positions is the point where the bicep becomes actuated to slow your punch, this happens in many areas throughout your body during the excecution of the punch (although, as anyone here will tell you, different punches will involve diiferent dynamics). So learn to relax the antagonistic muscles and be as aware as you can in practice of what might be negatively affecting your strike....oh, and don't forget torque....big subject...e-mail me if your want a dissertation.