How do I stop my saddle pad from slipping back?

When I tack my horse I always make sure that the saddle is on the pad and not too in front or in back of it (it is my biggest pet peeve). Because I know that it can hurt the horse when the leather of the saddle rubs up against there back. So I feel really bad when I finish riding and see that her saddle pad has slipped back a good 5 inches!!! How do I stop this?