Wow. A lot of folk wisdom, but rather than spit, coat, notch, or hold my breath, I found a better solution. My new Scorpion full face helmet [Snell and DOT, polycarb] comes with a fog-free shield. Much to my great pleasure and surprise.....THE SHIELD DOES NOT FOG UP. NEVER. COLD. HOT. RAIN. NEVER! [and, it is comfy and quiet].
And all for $140. After a few weeks using it, I put on both my old Shoei and Arai helmets and within minutes on a cold morning, they were fogged over. How'd I ride for 30 years like that!!!
Larry in Boston