Since winter's only just getting started, I thought it would be a good idea to review ways to protect cameras and gear from cold weather, and expand on the post I originally wrote several years ago.
The plastic bag method is still the best to prevent the formation of condensation. However, the best approach to reduce problems associated with extreme cold is to keep gear warmer than the ambient air. Since you're already wearing an insulating coat, keep your camera in there between shoots and put it back before it has a chance to cool off. Keeping the inside of the gear bag warm using a hand warmer is also a great approach. However, if the camera is cold and the inside of your jacket or bag is warm, condensation can also form, so keep that in mind and use the plastic bag method.
Besides condensation, plastic parts may become brittle and metal has a tendency to shrink, and film can become brittle. As a result, handle gear and film more gently, especially when changing lenses, and take it slow when rewinding manual cameras. For those of us still using instant film, note that development will slow as temperatures drop, so adjust times accordingly (this may require some trial and error until all the specifics are worked out).
I hope these help! If you have additional suggestions or tips for protecting gear from winter weather, please let us know in the comments.
Picture: West Virginia, 2010, Angela Kleis