The best tip is the one thing that particular skier needs to work on the most. Sometimes a hard part of teaching is deciding what NOT to say to help them. The tip for the single most important thing they need to work on is the best tip for them. Sitting back...gott'a get the weight properly distributed before anything else works right. Rotation of the upper body to bring the skis around...gott'a teach them to ski with their feet. If only one thing is learned by the student in a 2 hour lesson, but that one thing is critical to learn before anything else can work right, then the lesson was a success.
SkiMangoJazz, to add to the "pull your feet back," try telling them to pull the inside foot back all the time, and to pull both feet back when the skis are light on the snow. If they are dedicated sit-back skiers, convince them to pull that inside foot back HARD all the time.