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!
![EEK!](http://www.otca.com.au/boards/images/smilies/eek.png)
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.
![Wink](http://www.otca.com.au/boards/images/smilies/wink.png)
Of course, you can always use Thai pads too.
![Big Grin](http://www.otca.com.au/boards/images/smilies/biggrin.png)