What You Wear
Layering yourself in the right clothes is going to make a huge difference to enjoying your winter camping experience. The first rule of winter hiking and camping is, do NOT use cotton layers. A common saying among winter enthusiasts is "cotton kills." It doesn't wick moisture and when wet, doesn't insulate. Make sure you have base layers, mid layers and over layers. Base layers should be moisture-wicking and quick drying such as capilene, polypro or wool, mid layers should insulate, such as fleece, down or wool, and the over layers should be weather proof to keep out wind and moisture from penetrating the layers underneath. Down or synthetic insulated puff pants are also fantastic for winter camping. Protect your extremities with the proper attire such as hat, gloves or mitts, and non-cotton socks. Always keep a pair of extra-thick dry socks that you only use at night in your sleeping bag so that you always have a dry pair at night. Boots should be waterproof and insulated.
How You Sleep
Recouping at night can be difficult if your body is battling the cold. Use a one- or two-wall tent that is rated for four seasons. One-wall tents are a bit lighter. To create insulation from the snow while sleeping in your tent, place not one but two sleeping pads under your sleeping bag. Some prefer to use a shorter foam pad under their full-length inflatable one. Your sleeping bag should be rated the proper temperature for where you camp. Down or synthetic bags rated to -20 are sufficient for most winter campers, but if you sleep extra cold or will be camping in a place where temperatures are liable to drop below -20, then a -40 bag is recommended. Keep in mind that down doesn't insulate when wet, so if winter camping in a damp place, bring a synthetic bag. At camp, wear down booties to keep your feet warm and dry, and make sure to eat enough calories to help your metabolism combat the temperatures.
Backpacks for winter camping must be big enough to hold all the bulky clothing and extra gear. Most people find they need a backpack at least 7 to 15 liters larger than their three-season pack. You will of course need your kitchen gear, which should include a stove capable of handling cold weather such as the MSR Whisper Light or Dragonfly with a liquid fuel such as white gas, and a pot for boiling snow or water and a cooking tin. Other gear to have in winter is a small piece of foam (1 by 2 feet is better than nothing) to kneel on while manning the stove. This helps to insulate from the cook from the cold ground. This could be cut right from a sleeping pad roll. Water bottles are an important item not only to keep hydrated with during the day but also useful for filling hot water with at night and keeping them in your sleeping bag to act as a heater. Slip a sock over the water bottle so that the boiling temperature doesn't burn you in the sleeping bag.