Cadpat boonie or cadpat material bandana tied to my head. Depends on teh weather and my mood I suppose. My Bolle's don't fog up much from the boonie, but I DO have the problem of sweat building up on the top edge of the goggles and leaking down the inside of the vsor, creating the same type of problem, but harder to get rid of. Flip the front of the boonie up 'sniper style' and fog goes away fast. Sweat......... ugh! Wearing the bandana low so it covers my eyebrows, then putting the top edge of my goggles on that prevents sweat from getting on the goggles. When I wear my boonie I might just have to go for a Rambo headand to do the same thing.
I'm pretty old school, if that shows.
Ok, once in while I'll wear a ballcap as well.