I have seen goggles on the slope which have a small fan for air circulation mounted on them. Can't remember who made them though.

Edit: these are the ones

http://www.untracked.com/highlight-176522.html

The main thing seems to be to keep the vents clear. My Oakleys only generally mist up if I'm not moving - when I'm bombing down the hill they will clear very quickly. Mine has front vents as well as the top/bottom ones, which might be helping.

Hugo