+1 for the Polar Buff.
Simple and very versatile. I make sure the polar fleece end is down by my neck so it not only seals out the wind around my neck but the fleece material is full enough to close up the gap under the helmet. That still leaves me enough material to make a nice balaclava and protect my face. When I get off the bike it turns into a nice cap.
DL-650 Gone but not forgotten