rec.backcountry has an informative thread.

On the more concrete level: in winter camping/travel moisture is your
enemy and your goal. Be warm through layers; keep dry through not
overlayering. Be able to ventilate without chilling; be able to hydrate
without excessive bodily moisture. Do not carry heavy volumes of liquid
– find it in your surroundings. Have much liquid but moderate
carbohydrates in your diet and recipes. Cook for protein first, liquid
second and carbohydrates last. Heat beverages for internal body warmth.
Carry backups to all your layered clothing options. Carry double rations
of any medications/suppliments that are important to you, even the
1-a-day vitamins; your psyche needs them even when your body does not
and if there are unforseen delays you will be better prepared and
perhaps saved by any essential medications.

I’ve also heard it said that the best plan for winter camping is finding someplace dry to stow your gear till spring — but what fun would that be?