It doesn't get all that cold here in GA... but I've hunted a few mornings in the 20's and as I get older and more out of shape, I seem to get colder easier... I carry a cheap, old (like $30 one) sleeping bag with me on really cold mornings. When I get to the stand, I'll shuck my boots and sit inside the sleeping bag zippered up to about my waist. My feet stay nice and toasty. Obviously, you can't do this if you're stalking or still hunting, but, my feet don't typically get cold if I'm doing that anyway.

Cheers!
MadOtis