Here's some advice from Kristoph Thompson:
In addition to using the machines in the gym you can perform some exercises that require your body to move in the same way as when skiing. First try side lunges. Standing upright, take a large step to the right, bend your hips and knees slightly and push off with right foot and stand feet together again. Try the same only stepping the left, alternate sides and repeat 12-15 times.
Visit Kristoph's web site.
More advice about skiing preparation exercises.
