I don't know if you guys have seen or used these, but you can actually get focus pads which have purposely strengthened surfaces which sting and condition your knuckles -- some of them actually have a thin layer of ply wood underneat the padding!

My personal focus pads at home are made from this kind of really thick material that delivers a mild sting when you punch them. It's not enough to really hurt or anything, but it does strengthen your knuckles after you get used to them.

Of course, you can always use Thai pads too.
