When decorating a teen room I would do the room mostly white or ask the teen for her interests and likes. As a teenager you don't have a certain style; you change all the time - what's in and what's out and what is popular among friends... That's why I would do an all white room with maybe a little black or beige, so that the teen can change up the decor easily. Posters, paintings, pictures, pillows, candles can all make the room really personal and that's what teenrooms are all about. It has to be personal and a place she can enjoy and love.
A teenage room is like an apartment all in one room, so there have to be a lot of room for storage. There should be a bed, a desk and a closet and shelves for putting up personal stuff like pics and other things the girl loves right now. I love the floating shelves from IKEA. Instead of putting up pictures all over and hitting nails in all over just put your pictures on shelves - this way it's so easy to change and doesn't leave a lot of holes in the wall.
IKEA is a great place to go teenage room shopping. They also have a lot of cool little nig nags that are not that expensive and they have great storage solutions as well. Right now neons are really in, so if the teen love the neon-trend you could easily do little DIY projects to bring in a bit of neon - that also goes for all other colors. Doing DIY projects with your teen is really a nice bonding experience and she also gets some pieces for her room that noone else has and that'll be really special to her.
These two last room are of Paris Hilton's and Kim Kardashian's former bedroom. If your teen loves a particular celebrity then go find some pieces similar to what you see. Both Paris and Kim had mirrored furniture in their bedroom.