If you have a poncho or an emergency blanket, decide whether you're better off wrapping yourself in it or using it as part of the roof structure or your shelter to shed rain and wind. To make that decision, consider the ambient temperature--if it's comfortable, and your greatest concern is staying dry (say, in a tropical monsoon), use the poncho or emergency blanket as part of the shelter. If, on the other hand, you're facing a cold night, do the best you can to seal the roof with natural materials and wrap yourself in the poncho or blanket to retain your body warmth and stay dry. These two items are absolute lifesavers, and every outdoorsman should pack each. I carry a Heatsheets Survival Blanket, made by Adventure Medical Kits (adventuremedicalkits.com). My pocket poncho is an inexpensive vinyl model made by Coghlan's, available for a couple bucks at sporting goods stores everywhere.
With all of these shelters, pile dry leaves or soft boughs on the floor to insulate your body from the hard, possibly damp and certainly cold ground. You lose an enormous amount of body heat by contact with the ground, so anything you can do to insulate yourself from direct contact will help prevent hypothermia. To help keep warm, rake some of the soft debris over yourself to serve as an insulating layer. Sleep tight!