I cheat and have 2 "Johnny Handwarmers". One for each of the pockets in my parka. They look like big Zippo lighters for anyone who has not seen one. You fill them with lighter fluid, light them until the wiick glows, put them in their little felt bags and you're good for 8 hours. Between exposures, I either put my hands in the very warm pockets or slip the warmer into my mittens. Highly recommended for winter shooting!