Have you taught her to stand? I do that before I try to back them up. Have you tried using any type of a touch pad to teach her to hold her front still and move her rear? That can make it easier. You can work from directly in front of her luring her left and right, then back.
Another thing I've done is heel in left circles, smaller and smaller till she's backing out of your way to maintain the position but that one is a little more dependent on them really knowing they have to stay in that heel position.