They are a hard age to buy for, trends seem to go in and out so fast, something which is cool one week can be seriously uncool the next, I tend to go for jewellery aswell for DP's little sister who is at that adolescent stage, although anything I buy her would be considered cool, as to her I am apparently the height of coolness!?! follows me round constantly and agrees with everything I say! She's chosen a very bad role model!
What about some nice slippers? If you go on www.asos.com they have some white and pink fluffy slippers for £8 (in sale from £15). The only thing is, they're the playboy make so have small pb bunnies on the front, but my neice (12) had them off me for Xmas and she loved them (and was oblivious to pb connection).
Failing that, some lovely posh smellies - it'll make her feel more grown up than getting say Boots or Body Shop would. there's a site called cosmeticsdirect.com which sell that kind of stuff. Hope that helps.
Cool stationary (all matching bits) from paperchase went down well with my SIL who's 14. Plus lightweight so easy to post. Either that or body/lip gloss stuff. Having said that I bought her some body spray and stuff from natural collection once and she commented that it smelt a bit like her cerruti. Was very shocked as she was only 12 at the time!