My financially-minded but spatially-challenged kiddos grasped the concept of negative numbers as debts or bills and positive numbers as assets. This made the rules about signed operations a little more intuitive for them: taking away a debt naturally means that you are adding to the money you keep, for example. I've also seen students use the concepts of ladders and holes, with the zero point being ground level, too. A vertical number line might be more helpful for grasping this concept than one oriented horizontally. Presenting it this way ties in with talking about numbers being "higher" or "lower", which she is probably already familiar with.
HTH
