A good baselayer is essential for almost any athletic pursuit, especially winter activities. It moves moisture away from your skin and keeps cold spots down. Many layers are made to trap air against the skin.
The material of the baselayer is incredibly important. Most are made of a synthetic
material or wool. Cotton is avoided because its fibers tend to collapse when wet and it doesn't dry quickly, leaving wet spots that cause cold spots.
Fit is vital as well. A good baselayer will fit like a second skin, not too tight, but without wrinkles. It should not bind or restrict movement.
Another feature that's nice to have is an antimicrobial layer. Often some of the nicer baselayers will incorporate biosilver or some kind of substitute to inhibit bacteria growth that keeps odor in check.