Every body has their own version on this one...I wrote a paper on it at college in Oxford so I have spent a bit of time studying this one. Before you can analyse the factors governing any body mechanic you have to have an idea of what the mechanic is. Thus, you say 'punch' well, what punch are you talking about, is it linear (jab, cross and so on) or does it follow an arc (hook, uppercut) are you looking for a percussive response from your strike or concussive even penetrative. Each has a diferent wounding ratio and knockback. What is the target area and are you particularly tall or short...these will affect you possible angles of attack.
Also consider previous posts, if I put you on roller skates how good would your punch be? Ice skates? Ice, Marbles or in the sea? If I cut you legs off how would you generate your punch, would it be harder as a result of not having legs? Is your opposing arm of value? Remove the arm and what happens to the punch on the opposite side? Try punching whilst isolating each area of the body and seee the difference it makes when hitting a focus mit, with feedback from a freind. You can then see what factors are important in your technique and see overall where different power is generated from and work accordingly. Look at the technique iteself and which combinations lend themselves to better mechanics...it is true the position of your opponent won't necesserily aid your combinations but this sort of thoughtful training will give you an awareness of the body mechanics and fluid momentum required to enhance your technique...whatever it is on that day.
The fact is, most people have an opinion on how to hit harder and universaly, we talk about what has worked for us personally; to realy improve you must discover what works for you and this is best aided by experimentation. If you have a good grasp of anatomy, mechanics and know what, when and how you want to hit then you can work it out in theory...if you don't want to write a paper on it...then you can only learn through the feel of it.
Good luck mate and if I can help with anything specific it means I did not waste my time at University. Cheers.