Enemy #3: Accessibility
If it is all possible, keep your camera warm. With point and shoot cameras this is easy enough; just keep it in your pocket until it is ready to use. SLR owners, especially pro SLR owners with big telephoto lenses are not afforded that luxury. So the trick is to find ways to minimize the chill. There are all sorts of neoprene or rubber “wraps” for your camera, but their only real benefit is not getting your bare skin stuck to your metal camera. As far as insulating from the cold, these products do little to mitigate the effects. A method I have found successful involves a specific camera bag made by Mountainsmith called the Quantum. It is a daypack with a top-load camera holster built in to the lid. Trapping a pocket of warm air and fueled by the heat radiating off my back keeps the camera from freezing entirely, but not warm enough to get foggy. The camera is still within reach to whip out and shoot when a moose struts by. If temps drop into extreme negative numbers, radiated body heat can be augmented by pocket warmers sold at sporting goods shops.
Gloves are another thing to consider as to operate many of the controls and dials, bare skin is required. Lowepro and a couple of other companies make photo or shooting gloves that can be worn while adjusting a camera. Essentially, any gloves made of a thin but firm material can be repurposed for photographic duty. The drawback is that the thinner gloves may not be the warmest ones, so consider wearing them under your regular winter gloves or mittens.