I was taught that a hard punch is partly to do with strength, but mostly to do with technique. A straight , or curved, punch using your legs, back and shoulder muscles combined is a hard punch. If you are not using most of your body muscles in your punch, your technique may need adjusting.

do pushups.i have been doing them and i feel a difference in my punch strength now.

heredia can make you a "tyson" in 4 months =)

with many training