So you are about to camp out. This may be planned or unplanned depending on how you day has gone. Hopefully you have given yourself some good options – because there are many ways to sleep in the woods.
If you have a choice, you should always find somewhere safe, warm/cool enough and comfortable. Circumstances, though, may make this difficult. Let’s start with the minimum needed for survival and progress toward (relative) luxury.
Basic: You can extend your period of survival if you have a heavy-duty (contractor strength) garbage bag in your pack. It can serve as a poncho, vapor-barrier (under you), or cover (over you); filled with leaves and such, it can provide insulation and/or become a mattress. Like a trash bag, other basic gear includes a one-use space blanket (mylar sheet), multi-use space blanket (stronger and less apt to puncture or tear), tarp, ground cloth or blanket. (Mylar is the weight-to-warmth winner, as it reflects your body-heat, rather than absorbing it, plus it is waterproof.)
One step up: Even in fairly cold environments, one can survive (and even sleep) in a simple shelter. MSAR requires knowledge of two types to be qualified to respond to searches: the debris shelter (made entirely of materials found in the woods + a space blanket), and the tarp shelter (using a tarp large enough to make a canopy over your sleeping bag or bivy sac). (Historically about 90% of all MSAR camping occurs in either a debris shelter or a tarp shelter)
One-and-a-half steps up: A hammock will get you off the cold, damp ground and away from most of the crawly things. You can also rig a tarp canopy, if rain threatens. However, some folks have difficulty sleeping in hammocks – so try it out (in different weather conditions/temperatures) before you get on trail (and have a back-up plan if you can’t find the right trees to make it work).
Two steps up: Various types of tents can be used, balancing features and durability against their carry-weight and cost.
All the way up: Campers and RV’s provide maximum protection from the elements, from critters and from other people, but are obviously much more expensive and cumbersome than the preceding methods, especially in rough, over-grown terrain — they only get you to the trailhead.
Site choice: Ideally, your campsite will have many of the following attributes: