Lock down the camera (so that there is no camera movement in the shot). Have her doing whatever you need her to do in the frame. When you're done shooting here, shoot a shot of the background she was in (still without moving the camera, in fact, just keep it rolling without touching it while she gets out of frame). Then in post, do a slow dissolve between the shot of her and the shot of just the background.
that worked perfect. I spent so much time on my last short using layers to make my ghost effect that I was trying to make it work that way and your way is so clean Thanks