Try not to sweat in the bag. If you are sweating, you are too warm. Wear a base layer if it's really cold, but again, open the bag if you find yourself overheating. To keep moisture off the outside of the bag, if it's not made of Goretex, Pertex or eVent, a light bivy cover like a BD Winter Bivy will work. The BD is no longer made, but someone must have something similar. Don't wrap it in a space blanket or anything like that as you will just trap moist air next to the bag,