I've noticed it's easier all round if I've been power-walking for a few months before going. But yes, agree with Dosh that it's the thighs that are crucial - and calf muscles if you're going to be on long drag lifts. Try sitting against a wall with knees bent at a right angle - that's a good indicator of how much it'll hurt if you don't get prepared. Oh, and if you want to avoid that 'cardboard slats in the calf muscles' feeling when you're away, for goodness sake do stretching exercises before and after a day's skiing - particularly after!
DH has just reminded me that there's a good thingy on the Wii Fit that he used a lot last year. Trouble is, last time I logged on to the Wii, it told the whole room how much weight I'd put on since the previous effort. 