Go to your local discount superstore and pick up a bulk pack of disposable heat packs...... they are in the outdoor section with the hunting goods.
Step 2: Get cold
While out traversing the woods or hunkered down waiting in your tree stand you may find that having a heat pack just doesn't cut it. Sure your hands are warm but toes, lower legs, thighs and possibly your core is getting cold....
Step 3: Think arteries
Stick the heat pack in your groin next to your femoral artery and that will warm up one leg and toes quick. If your arm is getting cold put it in your armpit at the brachial. You also have the neck artery as an option.