Also consider: worn or cupped tires, low pressures, and wind resistance from the hand guards. Do NOT underestimate cupped tires, they really do affect it!
If you really want to solve it: replace the steering bearings with tapered roller bearings from Allballs or install a steering damper.
-Skip
Most people that are having the "normal" headshake issues are having it at speed. I think cupped tires are the likely suspect here. That is the only thing that has ever caused me any deceleration headshake on any bike I have owned. My last case the tires looked good and were balanced correctly but they had some light cupping. New front tire and everything cleared right up.