Tarp material is the first important consideration. They range from inexpensive plastic ones with flimsy grommets to heavy-duty Cordura with bomb-proof stitched loops. Plastic tarps are good for minimal use in the short term. However, plastic fibers break down quickly due to heat, light, and chemical exposure. This can occur even stored away in a garage, your pack, trunk of a car, or toolbox. It will serve you well to avoid these if you want a tarp to last a long time. For a good middle-of-the-road tarp, choose one made from ripstop nylon. Nylon resists sun and chemicals very well, and ripstop is much stronger than woven plastic. If your budget allows, then splurge on a Cordura tarp. Cordura does all the great things that a nylon tarp does, but is also much stronger, which makes it a great choice for improvised setups. The Personal Survival Shelter (PSS) series of tarps from Wildnerness Innovations are excellent choices for top-of-the-line shelters that can be used in several different ways.