You could get her a Pandora bracelet... and a charm that has meaning for the two of you to get her started. Pandora bracelets are the new version of the old charm bracelet, and they appeal to women of all ages due to their ability to be uniquely personalized by the choice of charms. Then for every special occasion afterwards, you could choose another charm to add to her bracelet. I know that a Pandora bracelet is high on my wish list, and I'm reminded of that every time I see a billboard or tv ad for them!
Try asking her friends if there's something she wanted but didn't buy, maybe because she had to choose one out of 2 possible things, when she was out shopping. Or try to remember if you saw something like this happening the last time you went shopping or window shopping. That would probably make a good gift and here's an additional tip, if she takes you to expensive stores, you should start saving now for her next gift. :)
ask her what she wants. she may have mentioned something she really wants. it would be more meaningful if you get her something she has asked for but forgotten about rather than some trinket she possibly doesn;t want. make it personal