I'd say that you need to follow through with both positive and negative reinforcement. Rib her a little about it when you notice a new rub of paint on her bumper (but don't get angry; make it fun, but get the message across.) Mention it to her when you haven't seen any additional dings for a while. Let her know that you're occasionally watching her progress. And, perhaps, make her either clean off her bumper or pay once in a while to have the paint scratches washed off.
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