Base Layer: This is the layer of clothing closest to your skin that keeps you comfortable (and, ideally, wicks away moisture). There is a variety of fabrics to choose from-wool blends, silk, silk blends and various nylon and polypropylene materials-but there's only one rule: no cotton. Cotton can be wonderful in the summer, but in the winter, it retains moisture and dries slowly. Also, when choosing tops, pick one that claims to be odor resistant. You'll thank us later.