Hmm, if it's strictly sex then I look for what I find attractive. Nice eyes, nice lips, not TOO fussed about body, but if she has a good figure is nice. How a girl carries herself is important, for example: You could have the most physically gorgeous female in the world but if she doesn't know how to be sexy or beautiful in herself, no point.
This is all speaking from a purely physical point of view.
-AC
Personally, as long as they put thought into it, doesn't matter to me what it is. Like, for my birthday Ush got me a book that he thought I'd like, judging by my tastes and stuff, and I loved it. So, figure out what kind of stuff the girl likes and pick something out based on that, and try for something that's not the typical jewelry or candy or whatever. If she likes to read, try a book, or a CD if she likes music.
Or flowers. I'm a sucker for flowers. Even though I'm allergic to pollen.