Couple things. First, make sure you have a good fitting boot. You'd be surprised what kind of problems under your foot will cause shin bang. Second, put in a heel lift. It doesn't seem like a 1/4' thick piece of rubber would do much, but it does. Third, your boot just may be too stiff for freestyle. Stiff boots+big landings=mega shin bang.
Eric Peterson
Freeski Specialist
Tyrol Ski and Sports
Rochester, MN