That is a big question ! There is an article that you can get to from the Fighting Arts home page on the correct alighnment of a punch, this would be helpfull even if it is not exactly what you are looking for.
Some other things you may wish to bear in mind is that a punch does not just come from the arm, you need to put some hip movement into it to get full power and the correct alighnment of your body makes a difference as well