If you love the mountains and love the snow, winter camping can be the best camping of all. It's uncrowded, quiet, easy to track wildlife and maybe the most beautiful time in the outdoors. It is, however, quite a bit different than summer camping due to the extreme nature of the conditions.
Keep in mind that winter camping in the Northwest is not always SNOW camping; it can often become very cold rain camping, although you will definitely be sleeping on the snow. For this reason, it is important to come prepared for every kind of weather from 40º monsoons to 15º blizzards. A good rule of thumb is to bring all the stuff you normally bring on a summer hike, plus all the stuff you normally take skiing or snowboarding. See the Ten Essentials List and the Backpacking Checklist, then add the following: Waterproof, insulated boots (snowboard boots work well; don't even think about tennis or skate shoes), long polypro underwear tops and bottoms, extra wool-blend or polypro-blend heavy socks, waterproof/breathable coat and pants (again, snowboard or ski clothes work well), an extra fleece coat or vest, insulated gloves or mittens (a second pair of thinner fleece gloves that you can do camp chores in is a good idea, too), a knit or fleece cap that covers your ears, and a pair of gaiters if your snowpants don't securely cover your boots.
Additional Considerations: Since dryness is essential to staying warm, make sure all the insulating layers you bring are polypro rather than cotton. You may want to use some restraint in your snow handling/groveling activities, since once your knees and gloves soak through, they won't dry out until after you get home. Take extra care to keep your boots dry and free from snow (they'll get cold anyway) when you get ready to bed down by making sure they are under cover or inside your tent (they won't be dirty). Also, be careful to keep as much snow as possible out of your tent – it can melt into water during the night and soak your bag.
Think about where you will be going, and what route you will be taking. If you are camping close to the car, or will be walking on a busy cat track or trail, your regular hiking boots may be just fine if it hasn't snowed much recently. On the other hand, if where you plan to go is off the beaten track or far into the backcountry or if there has been substantial snowfall in the past few weeks, you will probably need snowshoes or skis to get there. Keep in mind that you will need more surface area in deep, fresh snow conditions (longer snowshoes or skis) with a full pack than you would with a light daypack. Even if you plan to stay on a service road or trail, don't forget your map and compass – it's relatively easy to lose a trail in the winter and once you do, everything tends to look the same.
Make sure your sleeping bag is rated to 20º F or colder , and fluff it up before you use it to maximize its loft. Experienced winter campers usually prefer 0º bags, but you can make a 3 season bag work if you wear your long underwear (or more) inside it. Try to bring a full-length sleeping pad if you can; if not, you will have to fold some of your extra clothing under part of your body to insulate yourself from the snow.
Bring a 3 or 4-season tent if possible, and extra cord to guy it off (stabilize it by tying the ends and sides down). A lightweight summer tent may collapse under the weight of a heavy snowfall. If you have a lighter tent and it's snowing hard, you can make it work by setting your alarm and waking up every two hours or so to knock the snow off it (make sure you bring a watch with an alarm!)
Many stoves that work well in summer temperatures, especially canister fuel stoves, have inadequate pressure in cold weather and don't perform well for snow camping. A white gas stove with a primer pump will usually be your best bet. (You can prime a cold canister by warming it's base with a lighted match or lighter in very cold weather if needed). In addition, a base or support (a thin plastic cutting board works well) to hold the stove off the snow can improve its performance.
Since there is a good chance that all your water will have to be melted from snow, it is a good idea to plan your meals so that there is a minimum of cleanup (none is best). Many winter campers stick to no-cook meals (power bars, chocolate, beef jerky) and limit their hot foods to drinks.
Use common sense when selecting your campsite. That nice clear area at the base of the steep hill where all the tree stumps are broken off is probably a known avalanche path, and many snowcampers die as a result of being trapped in their tents by slides. Stomp around the area where you plan to pitch your tent and make sure there are no air pockets to collapse in the middle of the night, and look overhead to make sure no large accumulations of snow or ice (from trees, etc.) are likely to fall on you.
Be sure to bring extra batteries. All batteries have decreased life and efficiency when cold, and you will need your flashlight or headlamp more than you would during a summer overnight. Be prepared!
Bring extra fuel. Carry more fuel than you would for a comparable period of summer camping – cold weather means everything you cook will take longer, and you are depending on your stove to melt snow into water!
A snow shovel can be very useful in preparing your tentsite or working with snow shelters. If the area in which you will be traveling/camping shows potential for avalanche activity, all members of your party should be equipped with transceivers, shovels and avalanche probes, and be trained in their use.