If you have to be out in the cold for an extended period of time, one way to warm up is using a heat tent. To make an emergency one, carry a lighter, a candle and an extra large poncho. And don’t be dumb about this, it’s dangerous, so pay attention and make sure the flame is extinguished before you sleep.
Pile a layer of insulation on the ground. Leaves and branches work well as long as you are extra sure to keep sparks away from them. Sit down on the insulation pile and position your extra large poncho to totally enclose your body, including your head. Light the candle. In this small space the air will quickly heat up. Be very careful to keep the poncho away from the candle flame and allow a small opening for fresh air. And don’t forget to put that layer of insulation under you; this is vital. Oh, and watch for sparks — the insulation can quickly turn to tinder.