When in doubt, consult her friends.
Consider running your choice by one of her close friends before you buy, particularly when buying items where style is involved, i.e. clothing, jewelry, etc.
Chances are, they know what her taste is better than you do. Again, don't ask them what she wants (she'll be disappointed if she finds out your great idea was actually her best friend's). Just run a few of your ideas by them, and ask which they think will work best. Then, if she doesn't like it, you can blame it on them! Just kidding.