Tween girls aren't into PINK PINK PINK any more, and they're beginning to be super-aware of what's cool and what isn't. The biggest sin is to buy presents that are way too young for them, so think hard before you give in to anything too sweet and fluffy. (Although it does, of course, depend on the girl. There are 18-year-olds with bedrooms knee-deep in big-eyed Beanies.)
High street shops like Lush, Claire's, H&M, Accessorize and Top Shop are good places to start, especially if you're willing to get chatting to the staff (who will happily tell you what's popular with tweens).
If you feel you're taking a bit of a risk, check that she can exchange your gift or get a refund – and make it absolutely clear on the gift tag that you won't mind in the least if she does.