hard punch

[Natswoo]

Best way to increase punch power is to improve your accuracy and technique.

[TommyD]

I've seen a lot of tips out there but I'm surpised there's nothing about trunk twists with a medicine ball. A lot of the momentum for your punch comes from rotating your trunk...

[carcajou]

...other things to focus on while trying to improve punch strength:

Aside from tricep and shoulder training...
-stronger legs
-stronger core (abdominals)
-your left hip drive for a right hand punch (right hip drive for left punch)
-punch straight...no wild 'haymakers'
-punch 'through' your target not 'at' your target