You must have shelter each night. Consider shelter to be the number 
one priority for survival. Have something that will protect your body from rain, snow and wind. Even if you have nothing else going for you—no fire, food, etc.—if you have adequate shelter, you have a good chance of surviving until rescuers come to get you.
Choose your campsite with care. Consider the following factors:
Avoid avalanche or snow slide areas.
The site should be protected from the wind if possible, but also near a large clearing suitable for ground to air signaling.
An overhanging rock shelf makes an ideal shelter.
In timber country where snow is deep, removing the snow from under a spruce tree will provide a quickly available shelter. Branches at snow level form the roof and branches may be cut and placed around the edges.
It can be helpful to dig a snow cave in winter where there are snowdrifts of sufficient depth. It's best to build a one-man cave about 3’ wide, 7’ long and just high enough for comfort. Don’t make it any larger than necessary. Snow caves are difficult to dig without getting wet, so they're less desirable than other forms of shelter. However, they do offer excellent insulation. Be certain to maintain ventilation in your snow cave by making a hole in the roof, and arch the inside roof so water will run down the sides. The sleeping shelf should be a foot or more above the entrance and covered with boughs or a ground cloth. Keep in mind that water and dirt destroy the insulating properties of clothing, and water conducts heat away from the body 27 times faster than dry, still air.
A lean-to is a good option in timbered terrain. To build one:
You'll need support, perhaps two upright poles or crotches in trees, in order to place the ridge pole about three feet from the ground.
Lean limbs or branches, butt end up if they have foliage, against the ridge pole.
Secure the lean-to and increase water resistance by interweaving cross members.
Thatch the roof with spruce or fir branches by placing the butt end toward the top. Complete the ends in the same manner.
Improve the lean-to by banking it with snow or soil.
A fire between the lean-to and reflector made of logs, stones (not from a stream bed, they may explode when heated) or a space rescue “blanket” will warm the shelter.
Plan for the worst first. Conserve and build up all resources right from the beginning, before greater emergencies arise. Disregard the probability of early rescue; build the most secure and comfortable shelter possible, one that will require little maintenance once it is constructed. The importance of doing the job well while you are able to do it cannot be overemphasized.
Clothing
Clothing is the most basic form of shelter. Dress for the weather – remember that it can and will change. Layers of clothing (shirts, vest, sweaters) are best. Be sure to add a layer before you become chilled and take off a layer before, not after, you become damp from perspiration. When you're wet, whether from outside climatic conditions or from perspiration, you are in trouble.
Good headgear is essential. You lose up to 45 percent of your heat around your head, neck and shoulders. Winter headgear should conserve heat, breathe and repel water. Summer headgear should ventilate and provide shade.
The body radiates heat readily from the head and extremities. The old saying, “if your feet are cold, put your hat on,” is good advice. Good headgear, footwear and gloves or mittens are absolute necessities to help conserve body heat. Keep clothing clean and dry.
Footwear
Choice of footwear is critical. Choose boots suitable for the terrain, weather and amount of walking to be done. Regardless of boot type, it is important to have a change of socks (preferably wool) to prevent dampness from perspiration condensation.
Sturdy leather boots are excellent footwear. Waterproofing used on leather boots should permit the leather to breathe. When boots become wet, dry slowly and carefully. Walk boots dry if conditions permit. Leather lined and insulated leather boots are extremely difficult to dry.
Rubber pac-type boots with removable felt liners are excellent footwear, but felts are difficult to dry when perspiration soaks the outer layers of felt and the inside of the boot. Some prefer the nylon or leather-topped rubber pac with removable felt liners. The leather-topped rubber pac is superior to the all-rubber pac, especially for walking, because of the breathing qualities of leather.
Gloves or mittens are a must even if weather is not cold. Gloves will prevent injury to the hands when breaking firewood or building a shelter. In extreme cold, mittens are superior to gloves.
Carry some type of rain gear. Remember, waterproof clothing does not breathe. Movement should be kept at a minimum when wearing rain gear to lessen perspiration. A raincoat, water-repellent parka, or poncho should offer some ventilation and ease of movement.
