If you have to spend the night in the woods try to make shelter before dark - the shelter will help protect you and keep you warm. Try to find a tree with a Y shape in it and then another branch you can lean up against it. Once you have that frame, find other branches you can lean up against the long branch resting in the Y. Then take dry leaves and branches and fill it in as much as possible. Clear the area under the shelter of rocks and such so it's more comfortable. Then lay down a layer of brush and leaves, etc. You can also stuff dry leaves inside your clothes to help act as a layer of (itchy) insulation. Anything to help you stay warm! If your with other people - it's time to get cozy! You want to maximize each other's body warmth.