Much of this information is taken from the Canadian NCCP level II technical manual. Each period is a standard 4 week period.
With any of the above recommendations remember there is no right of wrong number of sets and reps. Remember the basic principles: exercises should be performed to local muscle fatigue, exercises must be performed with snap, and you must get adequate rest between sets. Try to be as ski specific as possible.
Exercises include (for further ideas check out your local bookstore or library):
Back-Ups (Roman Chair)