I always carry several unopened pairs of Hot Hands in my backpack. Typically, the hike in keeps me warm for the first hour or so, but when extremities begin go get cold, I'll fire up a pair of the Hot Hands and stick 'em in my gloves. I also carry a couple of sets of toe warmers, which I will use in the same way. However, I've not hunted in conditions cold enough in the past couple of years to need the toe warmers inside my pac boots.
Cogito, ergo armatus sum.