Being an old cheap fart when i was young, I borrowed a couple of those jet fuel fire in your pocket things and had them leak. The rash i got from the leaky bugger, needed no additional heat. My wife always buys the chemical hand warmers and never uses them, we have drawer with a bunch in them somewhere around here. She had trouble getting them open in the field with the tear open plastic, with cold fingers. "you got a knife?" I asked. "My fingers were too cold to get my knife open, and my feet are so cold that i can barely walk, let's go home." She needed three things, a new knife, better gloves and warmer boots. Now she has no need for the hand warmers and forgot where she hid them from me. I hate wearing heavy gloves and I like to wear unlined canvas hunting boots as I never am having any fun until my feet are wet.
My low back can get really mad at me sometimes, especially when i have been sitting on my Nifty Seat on a cold day for a long time. I find that a couple of chemical hand warmers under my wool shirt in the right spots on my low back really help keep those low back muscles from binding up.