As many people stated before, strenthening up your neck will help you tremendously with absorbing impact to the head. Mike Tyson used to work very heavily on strengthining his neck, if you ever look at old training footage of his, he used a neck harness.
In addition, I've seen some videos by Walter "Sleeper" Michalowski, former world muay thai champion. It deals with strengthening the jaw by placing your own pressure against your chin and try to open your mouth. The videos deal with some other drills as well.
