Well, then my friend, you ought not be asking around here. You know her better than all of us put together. I'm sure you can come up with something special.
Well, one of my best friends is a female, and on her last bday i gave her a giant fluffy teddy bear in pink trust me she loved it, and by the next day all her closest gfriends knew about it My advise - do the same, you won't regret it
Maybe get her an ITunes gift card, or a gift card to a store she likes. But if you want something more personal (and have several weeks) then you might think about making a scrap book with mementos and pictures of the two of you or something.
You have to decide for yourself what is appropriate, but we can at least throw out suggestions. Here's a few ideas I've used/thought about:
Stuffed animals (build a bear?)
Tickets to a concert/sports game/any event
CD mix
jewelry
book/CD/DVD
perfume
chocolate/food
a pet (nothing excessive - maybe a fish?)
lingerie
tickets to see a musical always go down a treat if they are into that sort of thing
jewlery but depends if you want to go out with her or not after this cos i would say jewlery is a gift you buy for someone you are going out with or want to in the near future
if you wanna go out buy her a heart shaped locket and have her name engraved on the back she will love it this is what i bought my gf for her birthday and shes worn it everyday since
something thats not dirty ... hmm. id say if shes a close friend then maybe something personal. like a photo in a nice frame with you 2 together then like some of that other crap to go with it ^