To prevent a foggy visor, I've tried the Fog City inserts, a lot of different sprays and wipes, and even spit. The inserts scratched too easily and distorted my vision on clear days, and the other stuff just didn't ever work for me.
I saw this at the local Cycle Gear, and decided to give it a shot.
Respro Foggy Breathguard It fits in any full-face helmet, only takes seconds to install, removes just as quickly, isn't uncomfortable at all to me, and works great. It just fits inside the helmet and forms a soft barrier between your breath and the shield. I used it on the way home from NY through the dense fog on a cool morning, and I recommend it to anyone w/ a fullface helmet.
Will