Tent Ground Sheet
A sheet of heavy plastic under your tent floor will help it last years longer. It shields it from all those little rocks, roots, pine cones, and sticks that you didn’t even know were there until you slipped off your air mattress or sleeping pad in the middle of the night! Just make sure your ground sheet isn’t sticking out beyond the tent walls, or it will gather rain water off those tent walls and channel it to under your tent floor, where it will eventually soak through.
There are wonderful commercial products available (http://www.rei.com/search?query=tent+footprints), but big spenders like me use a piece of heavy-gauge poly construction plastic sheeting, cut just a little smaller than the footprint of my tent.