From the FAQ...
How do I condition my shins?
To make your shins tougher and more tolerant of pain: Kick the heavy bag, kick the Thai pads, spar while wearing shin pads, or try light sparring without shin pads. Avoid the following: Rolling bottles or rolling pins on your shins, hitting your shins with sticks or other objects, kicking trees. (Yes, I know, in the past Thai boxers used to kick soft, sapling banana trees.)
My shins are really bruised and they hurt. What should I do?
Apply ice/cold to help get the bumps and contusions down. Then later, apply heat. An anti-inflammatory such as Advil can also help reduce swelling and help minimize the pain. Contrary to what some people say, Thai liniment does not help alleviate damaged shins; it is used to help you warm up prior to training or fighting. It is also useful for massage.