I always recommend doing alactic speed/strength/power concurrently during the aerobic base building phase. Workout of choice for XC is short, steep hill bounding. See pages 230-231 in Training for the New Alpinism for an explanation of this type of workout. Skiers should use poles for this. Short RS speeds another workout and useful for building economy at speed and power at speed. But be careful, falling on RS hurts.
Scott