Ah yes, winter riding. It's a novelty/adventure sort of thing now, with the first snows here, but in 3 months it will get really old. But you can’t just stop riding, now can you? I may not have learned how to jump much or get over many log piles in 10 years of mountain biking, but I sure have learned how to dress. After an early encounter with hypothermia - cotton is not a good idea if it’s 50-55 degrees and raining - overdressing was my rule of thumb for a few years, but now I’ve got it down (or so I tell myself when heading out in miserable weather). This lesson is especially important for women, as we tend to get cold more easily; and for cold wimps like me who are sitting huddled next to the space heater even though it’s about 68 degrees in the office.
It's best to go heavy on the feet, hands and ears, and not too bulky on your legs and arms. When you get seriously cold your body’s response is to restrict blood flow to the extremities and concentrate it at your internal organs; the airflow produced by your fast movement increases the heat lost at the outer reaches of your body. Yet you don’t want to wear too much on your trunk, since you aren’t playing shuffleboard after all, and will need to have unrestricted movement. Plus you don’t want to dress so warmly that you sweat excessively, as this will only chill you once the sweat starts to evaporate from your skin.
For this reason synthetic, sweat-wicking layers are what works best as the first layer, followed by 1-2 layers of stuff to trap warmth, such as fleece or wool. The top layers should be what stops the wind and water from getting to you, but still allows moisture from inside to evaporate out. Cotton is bad: it traps moisture and can’t retain heat when wet. Wool is good: the shape of the fibers traps heat even when wet, and it has natural wicking and waterproof properties.
I have a couple fleece sleeveless undershirts I use for below-freezing weather. I only cover my head if it's below 25 or very windy, as your head releases the most heat and you'll get sweaty quicker if it's covered. I usually use a headband or Buff, or for real cold a balaclava that’s thinner on top and thicker on the bottom. It’s tough to find gloves that are warm without being so bulky that you can’t shift or brake; the “lobster-claw” style works well, the “spork” of gloves, half mitten and half glove. Shoes can be tough as well; if you’re committed to riding in winter, invest in a good winter-specific pair that is larger than your normal size to accommodate extra layers of socks.
Oh yeah, in general dress warmer for road rides than for mountain biking, as on the road you'll be more exposed to wind, and going faster with less effort, especially downhill.
If you go on a big group mountain bike ride, you may want to dress more warmly than for a solo ride, as there will probably be more stops. That has caused me trouble in the past - I've dressed for riding, not for standing around watching the 3rd flat get fixed. Take an extra outer layer on a group ride, and don't be afraid to stop to put it on or remove it as necessary.
Bonus - it doesn't hurt as much if you crash with 5 layers on!
