Heat exchanger facemask or balaklava is the best solution I've found. Balaklavas or scarfs get damp and icy, plus they really can't warm the air up much. A heat exchanger uses the warmth you just blew out to heat up the air you're sucking in. Usually, I get colds as soon as winter comes, from breathing hard in cold air, but last winter I rode with the heat exchanger and never got a cold. Check it out at:

http://www.psolar.com/