Buy a neoprene one. Conventional cloth masks get soggy and lose effectiveness quickly; a thick neoprene mask will keep the wind off your mug and retain heat while maintaining its effectiveness. I've used a mid-sized mask when skiing for years, it's an absolute miracle. Pic related, similar model.
You can get ones with a neck gaiter extension too, for when it's seriously cold.
http://www.zanheadgear.com/catalog/product.aspx?ItemCode=WNFM114N&CategoryID=101