BATTERIESBatteries don't like cold weather. Cold temperatures reduce battery performance, so you should take measures to keep your camera and spare batteries warm.
Keep the spare batteries in a warm pocket (in the layer of clothing closest to you). Make sure the protective covers are in place—you don't want the unprotected battery terminals to come into contact with your metallic keys or loose change in a pocket. If you can devote a pocket solely to batteries, that would be great. Fully charge all batteries before heading out into the cold.
Carry your camera under your coat except when you're actually shooting to keep it warm. If camera performance falls off, switch to one of your warm spare batteries, and put the battery from the camera into a warm pocket to thaw.
Lithium batteries perform better than other types in cold weather, so use them if your camera permits. Most dedicated rechargeable batteries for DSLRs lge lithium-ion types these days, and you can get lithium AA cells for cameras that use that type of battery.
MEMORY CARDSMost of today's major-brand memory cards can be used in temperatures down to freezing, and some higher-end cards can be used below that. Some cards are even waterproof (Hoodman's RAW Steel SDHC and SanDisk's Extreme Pro SDXC 8-32 MB versions, for example), although presumably even a nonwaterproof card would be protected inside a weatherproof camera.
Have a dry cloth handy to keep things dry if you need to switch out memory cards. Don't change memory cards or batteries in the rain—you must protect even a waterproof camera from moisture while it's open.
|IF YOUR CAMERA ISN'T
For the best waterproof protection, you might consider an underwater housing for your DSLR. Designed for serious underwater photographers, some of these can protect your gear at depths down to 200 feet below the surface. However, these are specialty accessories and can be very expensive.
If you don't plan on doing underwater photography, you probably won't want to invest in a complete underwater housing. In that case, a simple rain cover such as the ewa-marine Rain Cape or the Kata Rain Cover or Camcorder Glove is just the ticket. These provide protection from rain and snow, but make it easier to operate the camera controls than the true underwater housings that completely seal the camera. ewa-marine also makes low-cost, flexible underwater housings that allow you to completely submerge the camera, but these are more for underwater photography—and it's more difficult to operate the camera controls than with the Rain Cape and Rain Cover.
A number of camera manufacturers offer waterproof housings for some of their compact digital cameras (and even DSLRs), and these provide good weather protection; they even allow the cameras to be completely submerged in water to shallow depths.