Punch with a firm wrist and a loose fist. NOT the other way around (as so many people do).
Tight fists lead to broken bones (and not for the other person).
Practice on something soft like a mattress until this technique becomes automatic. Your punching power will be increased tenfold!
A loose fist gives the hand considerable shock-absorbing ability. A firm wrist is necessary as well. A great puncher will NEVER bend his wrist upon striking a target the way most women and boys do.
When the wrist is firm, the fist AUTOMATICALLY takes on a degree of firmness. Moreover, keeping the fist plyable will allow for maximum punching power--A firm fist is a broken fist.
Once a person learns how to make a proper fist, he or she can then strike using all of his or her body weight.