I used to have this problem when I rode spks years and years ago. Here is how I solved the problem:
First off I was in boots that were to big so I went to a fitter and got into some smaller ones. Bring your boots to a fitter and see if there is just a bit too much room in the ankle / calf.
When bang still persisted I switched to a wrap liner which provided a bit more cushion in front which helped allot. I went with an intuition wrap but there are others out there.
When this didn't solve it all I switched to a softer boot (the 90 flex spk forget what its called). Which helped allot too.
Here we get into rough territory when it comes to fitting. I switched to a three piece boot (dalbello / Full Tilt) when things weren't solved. Here is why when flexing the cuff of a three piece boot it will conform more to the angle of your leg in comparison to your ankle.
I have friends though that went in the opposite direction choosing a stiffer (120-130 flex) race boot so instead of bending the boot they bent the ski when they shifted weight to butter. For me that caused more shin bang for them they say it solved the problem.
So Spark Notes:
1. Go to a fitter see if your boot needs fitting.
2. If the fitter thinks it will help try a different boot liner
3. Try a stiffer or softer boot. (find one that fits you at the store bring your skis and just butter on the carpet to see how the different flexes feel).
4. Try different boot designs (two piece vs. three piece).
Do this all with a boot fitter.