At wally world you can buy boot inserts that have a little slot under the toes. Take out the foam insert when it's cold and insert those little foot warmer sacks (I think foot warmer might be the brand or Hot Hands?) Anyway -- you're feet might still sweat (though some of the tips mentioned are worth a try) but the controlled rust chemical action generates heat for 6-8 hours. They are vunderbar. You can just drop one of those packets in your boot, but it's always sliding around and finding the most uncomfortable part of your foot to annoy -- best to use the boot inserts.