Winter backpacking can mean your footprints are the only ones out there. That adds to the beauty of the experience... but also to the danger. Alone and in a cold environment, it's important to know what to do in an emergency.
Learning a few basic cold weather survival skills can save your life, so take a minute to read them now.
Imagine slipping into a stream and soaking everything with you, when you are more than a day from the nearest road and it's below freezing outside. What would you do? Start a fire, of course, but can you?
Always carry waterproof matches, and practice with starting a fire in the cold BEFORE you go winter backpacking. Learn which tinders work even when wet. Birch bark, for example, will burn when wet, and so will sap from pines and spruces. You may have only minutes before your fingers get too cold to function, so speed is of the essence.
You'll probably have a tent with you, but you still may want to learn shelter building using snow blocks. Sometimes you can stomp out blocks without tools, using your feet, and then lift them from beneath. Just play around in your own backyard until you get the hang of it. In an emergency, or if the weather turns extremely cold, you may want to put your tent behind a wall of snow blocks, to stop the wind.
If it isn't raining, you'll want to build a quick survival shelter for warmth. For that you'll need any of the following: a pile of dry leaves, grass, broken ferns or other plants.
I once collected enough dried grass from a frozen swamp in thirty minutes to make a pile several feet thick. I slept warmly in the middle of it (half the insulating grass on top of me, half of it underneath) with just a jacket, despite below freezing temperatures.
You can be wet and warm when it is far below freezing, as long as you are active. The moment you stop moving, however, you start to lose your body heat. Once you get chilled through, it is difficult to get warm again. Hypothermia (a lowered body temperature) kills many people every year.
If you get wet, try to get dry before you go to sleep. Put dry clothes on if you have them, and use a fire to dry any wet clothes. Earlier in the day, you may be able to hang damp clothes on your pack to dry in the sun. Often when it is coldest, the air is dryer.
Try not to sweat. Adjust your layers, removing and adding shirts, sweaters and jackets as necessary to keep from getting too hot or too cold. Sweat, and clothes damp with sweat, will cause you to lose body heat fast once you stop moving. Stay dry to stay warm.
There are many other cold weather survival skills that you may want to learn. (You can generate heat by eating fatty foods, for example.)
You don't need to know hundreds of skills and techniques, but why not learn a few basics, like the ones above, before your next winter backpacking trip?
For more learning, check out the "Winter Camping Basics" article from REI.