We need to clarify what we mean by a trash bag. We aren't talking about the small white 13-gallon bags that you use under the kitchen sink. We are talking about the 42-gallon lawn and leaf bags that you buy at home improvement stores like Lowes and Home Depot. These heavy bags have the durability that is needed to survive the abuse you will put them through out on the trail. In recognition of the important role the lowly trash bag plays in the outdoors, we submit for your entertainment ten suggestions on how you can use one on your next adventure.
Make an emergency poncho. Although it does have one disadvantage that it won't breathe at all, in an emergency it beats the alternative of hypothermia. If you are in a light rain and you are interested in keeping the water off your body, simply tear a hole for your head and stick it on through. If you need to maneuver around you can tear out holes for your arms. In a heavy rain you can tear out a smaller hole just for your face (remember kids, don't put a sealed plastic bag over your head). Make sure you flap the bag from time to time to ventilate yourself and don't let too much sweat build up.
Make an emergency winter coat. The main secret to staying warm is wearing layers. If you are down to your last layer and still shivering cold you can use your trash bag to create another one. Make a hole for your head and two for your arms. Keep in mind it will work better if you keep your arms close to your body so if you can't get by with your arms in the bag, even better. Fill the bag with leaves, duff, lichens, or any other dry material that can trap pockets of air in the bag. Tuck the open end of the bag into your pants. The stuffing serves as insulation and the bag acts as a moisture barrier.
Make a moisture barrier for your natural shelter. If you are sleeping in a shelter you made from natural materials like a snow cave or a lean-to you can use your trash bag two different ways. If you are worried about rain or moisture coming through the roof you can use the bag to augment your thatching. Simply cut the bag so it forms a large sheet and position it accordingly. If you are worried about the ground getting wet you can use the bag as a drop cloth. If the bag isn't long enough then cut the seams so it makes a long sheet.
Use it as an emergency dromedary bag. You certainly can't fill a 42-gallon trash bag with 42 gallons of water, but it will hold a couple of gallons easily. If you're using a trash bag this way make sure not to over fill it, it can stretch and break due to the density of the water, rendering the bag useless. Also never use a trash bag for long-term water storage unless it is absolutely necessary. Some chemicals can leech into the water and make you ill.
Use it as a make shift bear bag. Just like with water, unless your food supply is potato chips, your trash bag is going to have a limited ability to what it can hold. With some careful tying with cord you can suspend your makeshift bear bag out of the reach of both small and large critters. The plastic will mask most odors, but because the bag lacks a seal, it won't be perfect. Also make sure that smaller critters can't drop down onto the bag and chew through the plastic.