I use those "Toasty Toes" warmers with adhesive, that stick under your toes, when I hunt in a treestand in very cold weather. I swear there are plumes of smoke coming out of my boots when I use them. They only last about 4 hours though. Don't believe the writing that they last 6 to 8 hours.

I also take an Advil before heading out. It really helps.