Snowplowing really is the way to slow yourself down (if you feel unable to jump out of the tracks, you can try leaving one ski in the track, while snowplowing with the other). When you snowplow, try to keep your weight back a bit, and push down and out with your heels. Actually, on a really steep hill, if you want to slow down, you can sort of lean backward while snowplowing, to really try to dig in. Once you get the hang of it, you can try hockey stops (I don't recommend that quite yet, though ). Once you get some time in on your skis, you'll find you feel more comfortable. If you find that you feel unsteady on downhills, try to remember to keep your center of gravity low, and lead with your hands (tucking downhills does make you go faster, but it also keeps you steady, and you are less likely to fall). By all means, if you feel like you are going too fast, step out of the tracks, which will allow you to maneuver your skis better. It might help to find a nice little hill where you can practice snowplowing at really slow speeds until you get the feel for it.

Practice, practice, practice, and you'll learn to love the speed!