one of my favorite things to work with people (especially if not dealing with a group), standing still, have them try to stand on their toes (obviously the boots limit this) - kinda like the idea of shooting a basketball on skis, but not a quick move. Then we talk about reaching with this movement, standing on toes, arms extended (so to prevent the reach with the arms and squat with the butt). Then, with you stationary, hold out a pole and have the student reach for the pole (standing on the toes, arms reaching out in front of them - big trick is getting both hands reaching) as you bring the pole from up hill to down hill (making the student turn around you - kinda like the idea of leading a tortoise with a carrot on a stick, just out of reach). Once they get that, take the pole away (reach for the phantom pole) - the whole principle is to get the hip to start moving into the turn, and creating forward pressure, instead of sitting back. Next is to stop reaching with the arms (put them back into a skiing pose, and work with the idea of reaching with the hip (you can have them revert to the phantom reach and think about connecting the hands and hips with a string if reaching with the hips doesn't really work at first). By now the skis should be working better turning, and you can just work on mellowing out he movements (especially if there is a dramatic vertical movement, but at least they shouldn't be in the backseat anymore).
Another real quick trick I like to use is to make a few turns with a huge wedge (let them sit back). Then make a few with the smallest wedge possible (again if sitting back, so be it). Then make a few turns starting in the biggest wedge possible and ending the turn with the smallest wedge possible. If the are sitting back the edge will not allow the ski to get flat so it will not be easy to vary the size of the wedge. If this doesn't get em forward. I then point out (and have them do it too), leaning back in a wedge gets the skis on edge, but our body is out of whack. have them try, standing still, open into a wedge, then flex into the boots (instead of leaning back), and you can generate the same amount of edge, but now you are forward. Reinforce this with a few slow speed turns generating the edge angle by flexing the boot, and then re-try the varied size wedge turns, chances are the results will be very different. This is another one of those drills that you are really only looking for a little of what you are teaching them to transfer to their skiing (optimally, activating the ankles, getting forward, and ideally, without saying it, getting their hips into the right place to allow the inside ski to get flat).