I wear a balaclava under my 661 full-face mtb helmet in the winter with my wrap-around prescription safety glasses. The glasses almost never fog unless I purse my lips and blow right up there. They let just enough air by to circulate well but keep the wind out of my eyes.

I use one of these in my motorcycle helmet, reduces fogging enough I don't have to lift my face shield at stop lights. I think we move too much air to be in an enclosed helmet though.
http://www.aerostich.com/files/images/1558_1sa.jpg
Respro Foggy Breathguard