seems to be a little misinformation floatin around this thread. i'm no doctor but i did kickbox for 13 years and there were some widely accepted beliefs pertaining to this:
There are two main factors when improving your shins for kicking. Nerve sensitivity (the deadening thereof) and trabeculi. Both are improved upon in the same way, kicking a bag over and over. Trabeculi are microscopic, thin, tubular, bone tissues which deposit and grow on your bones. When you break a bone, for example, the lump that heals around it is made up of this material -- primarily calcium, it's a highly rigid tissue. Muscles have no, or a very negligible, effect on strengthening your shin bones. These bones are all but exposed, the muscles do not provide structure to this part of your body.
The very best thing you can do to build up your kicking power, your ability to not break your shin and a deadening of the pain associated with a shin strike is to work a bag. If you don't have one... use something else heavy, but able to absorb the energy of the strike. Don't use something rigid like wood. a hanging bag of sand might be your best bet (i don't know, maybe there are better things to use -- i still suggest checking out online listings for a used bag), although i imagine that's gonna hurt like a bitch too, you're at least less likely to break something with the sand.