I wear a "Headsweat" bandanna under my helmet that almost always seems to keep me warm in the cold, and reasonably comfortable in the heat. Last year I purchased a pair of Slimstreamz ear covers that fit on the helmet straps. One purpose is to place iPod earbuds outside the ear canal so I can listen to music and still hear traffic/riding companions, but they also provide good wind and sun protection for my ears, which burn easily.
http://www.performancebike.com/shop/...tegory_ID=1340
http://www.slipstreamz.com/content.asp?subID=8