Skiing is a high level activity that benefits from good cardio conditioning, flexibility and strength. Nothing new in that statement and that applies to most sports even fishing (just not as much). Cycling, running, soccer and other sports all offer cross training benefits to a day on the hill, and also helps prevent many injuries.
Cycling has long been a favorite crosstraining activity since it is great for cardio, uses similar muscles specific to skiing and it appeals to the outdoor mentality common in skiers. An afternoon on your bike can be one of the best thing you can do for your snow skills but is there something even better? Yes!
THE UNICYCLE!
What, a unicycle? But what if I wasn't raised by circus folk? Can normal people even buy these things?
Yes. It doesn't matter. You can buy them here or just search the net.
The unicycle has all the benefits of cycling but it adds a great deal of balance skills. Jean Claude Killy used to ride one and recommended it for off season training. Unicycling teaches precision center of gravity placement which transfers to precision weighting on your skis or snowboard especially from front to back. Since leaning forward or backwards causes so many problems keeping well centered makes your skiing more fluid and in control, you'll also make more saves than before and keep your head out of the snow. I learned to unicycle one summer and my next season on the hill I had much more control, a lighter touch on the snow and a cleaner brighter smile (some claims may be exagerated).
No clown suit required
But I don't want to be a clown! Neither did Dumbo and fortunately you don't have to either. There are unicycle clubs around the world where they wear shorts and t-shirts. Try Unicycling.org for tons of info on learning, buying uni's where to go watch people uni etc... And once you get good try mountain unicycling, off-road uni's have become popular recently and you can learn about that on John Foss' site.