One mantra you'll often hear repeated by people who spend significant time outside is the phrase "Cotton Kills." While you may enjoy lounging in a comfortable cotton t-shirt, the same shirt could contribute to discomfort on a long hike as it retains moisture from sweat. Look for shirts made of high-quality synthetic materials, such as polyester, that are made to wick moisture to the outer layer of the shirt. In the winter, some types of wool also work well to retain heat without soaking in moisture.
When going out to hike in the cold, you may be tempted to wear the heaviest flannel shirt you can find along with the fluffiest winter coat you can get. However, you'll often be surprised at how much your body can warm up while exerting yourself in the coldest of weather. Dress in multiple lightweight layers that you can take off as you go, avoiding overheating.
Conversely, when hiking to high elevation in hot weather, come prepared with a jacket to put on at the top of a potentially windy, cold summit. You may also find a pair of hiking pants with zip-off knee sections practical to convert between pants and shorts.
A lightweight, brimmed hiking hat is a worthwhile investment both to keep the sun off your head in bright areas and to keep away bugs. Again, look for a non-cotton hat to minimize moisture from sweating.
While shorts and long socks may not combine into the height of fashion, socks are beneficial to protect your legs from scratchy bushes and insect bites. In addition, wool socks can help to keep you warm in colder weather. Don't forget to pack an extra pair of socks or two in case you step in a mud puddle!