Your primary shelter is clothing, so play it safe on excursions from camp and take a jacket and hat. On every trip, pack a synthetic or wool next-to-skin layer, an insulating middle layer, and a waterproof exterior shell jacket. If you need to rig a secondary shelter in the form of a lean-to, use man-made materials whenever possible. "It's much easier and quicker to build a lean-to with a space blanket or a tube tent with trash bags hung over a rope than with debris," says Lundin. The reflective side of a space blanket will divert the sun's rays in the desert and retain body heat in cold conditions. If you must use natural materials to build a shelter, make the walls thick and angled to shed water and provide insulation. The simplest shelter is a lean-to constructed against a tree trunk or rock. Soak up the heat radiating from a tree canopy, big rock, or another person. You can also make a lean-to suitable for sleeping by layering debris over a fallen log. See "Pockets of Weather" (Wild Things, August 1999) for tips on where to place your shelter to warm up or cool off.