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