You're stranded in the woods, night is approaching, and temperatures are dropping faster than Santa's britches now that Mrs. C. spikes his Wheaties with Viagra. You've got not much more than your wits and a wool blanket, but with a little work, that's all you will need to get through a sub-zero night.

Step 1 : Build A Fire

If possible try to find a site protected and out of the wind.
Dig a trench in the earth the length of your torso (hip to shoulder), making it a foot wide and eight inches deep.
Line the bottom with rocks, (try not to use rocks out of a creek as they might burst in the heat) toss in some brush, and start a fire.
Keep it ablaze for two hours, giving yourself ample time to roast some polar-bear shish-kabobs and producing enough coals to make a toasty bed.
COALS, is the operative word , not large blazing chunks of wood.

Step 2 : Cover it up.

Spread the coals evenly throughout the trench, and then cover them with four inches of soil. " If the hole has been dug correctly, your'll feel the heat through the ground in about an hour or so," says Dr. Ron Hood.
If you follow his Two-Four-Eight Rule - a two-hour burn for the coals, Four Inches of dirt on top, and Eight-Inch deep trench - you'll keep your keister warm without getting it scorched. ;-)

That wool blanket might smell like a wet dog in August, but it'll shed water and retain heat even if it gets damp, so suck it up and deal. Lay the blanket flat over the trench as shown, matching up the corners of the blanket and the trench.
If you should be fortunate enough to also have a Poncho or a Tarp just make two layers with the Poncho/Tarp on the bottom with the blanket where it will be next to your body.

Step 4 : Wrap Up Tight .

Lying diagonally with your head and feet at opposite corners, fold the bottom over your feet and pull the sides around, tucking in all the corners, until you resemble a human burrito.
Heat from the coals will rise through the dirt and get trapped in the blanket. Don't worry if a light snow begins to fall - that frosty layer will help you retain even more warmth.
Finally , pull the top corner over your head and turn up your body's furnace ; try dreaming that your're rolling around on the beaches of Gilligan's Island with Mary Ann. That way, if you don't make it,, at least you'll die with a smile on your face . ;-)