I use the "bouncing" thing that was mentioned, only I take it to more of an extreme. I make them hop.
The first thing I do, is a static exercise, having them hop in place. Then I have them hop just the tails. I ask them to slam the skis back into the snow as hard as they can. That good, solid landing is what you want. It means they are in balance. You can, if necessary, have them do the same thing in a gentile straight run.
Once you are on the hill, moving, start out with them on terrain that is groomed, and a very comfortable pitch for them. Make some nice, comfortable medium to long radius turns at easy speeds. Have the students follow your tracks, and ask them to hop at a certain point in the turn. I usually start with a hop when they are facing straight down the fall line. I then ask them to hop 3 times during each turn. Just before the fall line, in the fall line, and just after the fall line. Then I go to 5 hops, and basically have them making little hops all the way through the turns. Then I go back to just one hop, and ask them to only hop the tails of the skis. This is what will really put them in balance, just as it did in the static exercise.
The "jist" of all this, is that if they can hop the tails of the skis at any point during the turn, and land solidly, then they are in balance throughout the turn.
When they get this on the easy terrain, move them up to slightly steeper terrain and do the same thing. Hopefully, you have a hill that has some pitch changes, and can do it without having to go to a different run/lift.
We do this with kids too. We call them "pop corn turns". As they follow you down the hill, have them hop when you yell "POP!". They love it, and it works really well. I've also found that if the kids are pretty good skiers and ski intermediate slopes, I'll find little bumps and ridges, and tell them to jump off anything they can. Taking a little air is a great balance exercise. As the pitch gets steeper, their body will learn that it has to be perpendicular to the slope to be able to land solidly. Slamming the feet into the ground (try to make an earthquake!) amplifies any unbalanced position.