Join Date: May 2007
Location: Buffalo NY
Bikes: Gerry Fisher Nirvana, LeMond Buenos Aires
Go check out a local ski store. There are lots of hats available today that are designed to be used with Ski helmets. The tops of the hads are windproof, but thin and breath very well, and the sides are usually thicker and fuzzy to provide comfort to the forehead and ears. I got my turtle fur cap for around $20 at a local ski shop. It made the helmet fit a tiny bit tighter, but interferes remarkably little.
For heavy rain I also have a rain cover, but it tends to hold in a lot of moisture that is normally released by you head. This is a nice bit of additional protection on either rainy or extreamly cold days (I consider extreamly cold anything below 10F).