There are many modes of exercise that can help improve your skiing fitness.  That being said, if you haven't tried the slide board, it's a great movement that helps improve your lactate threshold and targets ski specific muscles (i.e. the adductors, quads, glutes, abdominals, etc..).  Try performing 5 sets of 60 second intervals with 60 seconds of recovery between sets after your next workout.  Increase your intervals by 1 set per week until 10 are reached and then add 30 seconds to each work interval after that. This will teach your body to buffer lactate acid more efficiently and allow you to ski more intensely for longer periods of time.  For a more customized workout check out My SkiCoach.