If I don't talk to you on a regular basis, if you're not even an acquaintance, I'm not going to notice any changes you make appearance-wise.
If you don't know this girl very well, start a conversation with her. You can just say hi, or you can comment on a book she's reading or something. Find common ground to start a successful conversation.
Another great way to catch her attention if you don't know her is to compliment her. Not on anything inappropriate, mind you. If she's wearing a nice shirt, tell her that. Note that you may want to avoid that comment if her top is pretty low cut.
If you do know her, then you can catch her attention with an outrageous change in appearance. This may not always get a positive reaction, though. Clothes may be a bit subtle, unless you go from wearing all black to wearing... I don't know, bright pink.