The snow's a-flowing where some of you live, and that makes skiers, riders, and snowshoers very happy. To prepare your legs for the excessive stress on your quads, knees, and shins, it's best to condition yourself for the downhill. There are a few commercial treadmills available that offer a decline option, but instead of springing for a brand new piece of equipment, you can just raise the back of the one you have.
To do this, place one or two two-by-fours under the back of the treadmill. It'll offer a slight decline, but it's enough to allow you to work your legs. It's best to do it before you hit the slopes, since you don't want to realize you should have trained more while cruising down the hill.
Of course, you can also train downhill outside if you don't have a treadmill. Run downhill, or if that bothers your knees, walk instead. I recommend doing intervals of flat, incline, flat, decline, etc. Training downhill will not only strengthen your muscles and joints, but it can also prevent injury or extreme soreness from a day on the slopes. When you head into snow with your skis, board, or snowshoes, all this training will allow you to spend more time on the mountain and less time resting at the lodge.