knuckles

If you box then strong knuckles should come naturally but iof you have too then the old Shaolin Monk trick is too punch wood (2x4 or trees).

 

Get on a hard surface (hardwood floor, Concrete) and do push-ups on with your fists.

 

Punching tree's is the ole redneck boxer's trick.

 

this is wat i do. go bareknuckle on the heavy bag.

or
http://www.mawn.net/ki55.htm

 

Be careful when strengthening your knuckles or you'll break your hand.