In reply to Luk e:
A cool tip for skiing (more for the down than the up) is to put your skis down on the carpet, strap into your ski boots, and "sit" until your knees are at 90 degrees. Hold that for a bit and go back up. Do the same to 45 degrees. You can probably do repetitions or whatever. I've also tried lying down backwards and standing back up, since that's something I long to be able to do on the actual ski slope. Its bloody hard sitting still, I can't imagine it going well while sliding...
Squats are fun I suppose. Lunges might be more useful for skiing? Also lots of hopping up onto chairs in contrived patterns (pliometrics?)
Also of course lots of walking with big bags.
You might be able to tell, I'm not very scientific in my training. but last year when I was stuck in an office and there was no snow on the ground over in California I did a lot of research to stop myself going totally crazy.