Whether it be a weekend hiking trip or an all out "SHTF" situation your choice of clothing will play a role and is worth some thought ahead of time. And, your first thought should be don't wear cotton… especially denim. Denim, which is one of the thickest forms of cotton has a number of downfalls. Denim has no insulation properties, so when the temperature goes down it won't hold your body heat and leaves you vulnerable to the eliminates. In fact, cotton clothing and especially denim has been sited to actually help cause hypothermia due to it's poor insulation when it gets wet from perspiration or other sources. Cotton has also been known to cause hypothermia in warm weather due to it's tendency to absorb water. When cotton gets wet it absorbs the water molecules which causes the fabric to no longer breath, making it slow and difficult to dry and easily leads to hypothermia. The absorption of liquids also adds weight to the clothing, which in turn and more weight being carried by the person wearing it. And the worst effect of cotton's absorption is that cotton has a tendency to cause chaffing, rashes, or even worse open sores from wet clothing causing friction against wet skin.
When choosing what clothing to throw into your pack the better option is always clothing made of synthetic materials, especially materials designed to wick moisture away from the body such as Capilene. Other great material choices include Polyester, Nylon, and wool Blends.