If you're sliding backwards down a snow or ice covered hill, then there is not much you can do at all. Just keep in mind that the front of the car is heavier than the rear and if you are sliding for long enough the front will want to swing around and 'overtake' the back. You can try your best to keep things straight and in some sort of control, but mostly, just enjoy the ride. Your now at the mercy of whatever is behind you.
If you can stop the car on the hill and need to reverse down, put the transmission into reverse and use light braking on the way down. Get it into reverse as quick as possible.
Sometimes if you find yourself sliding, you can actually give the engine a little gas to accelerate slightly.This will bring the wheels up to the speed they need to be at to regain traction. This technique works well on rocky or loose gravel hills, and sometimes in snow and mud. Its a very unnerving feeling actually speeding up in order to get grip and slow down, but is a worthwhile trick to learn.