J.K.
When I turned 40 my sister wrote me a note that said "40 reasons I love my sister", and then she listed a bunch of sweet things that are special to her about me. It was so sweet and thoughtful.
You could also make her a photo book of the two of you through the years. You can scan in photos and then use iPhoto to make a book. They turn out really cute.
I second Laurie's idea of something personalized. I also have a monogrammed pocket mirror that my (very thoughtful) sister gave me.