I'm in agreement with the "no gift cards" theory. It is impersonal. A "real gift" shows that you thought about the girl and chose something just for her.
Teenage girls aren't hard to please -- I have two at my house, and I buy for several more in addition:
You can't go wrong with simple earrings, and they're easy to choose -- silver is still more popular than gold.
Scarves are all the rage, and -- again -- you can't go wrong. You can get a heavy warm scarf, or a lightweight fashionable scarf -- either is popular with teens. Target has nice choices for reasonable prices.
Several other people mentioned crazy socks; mis-matched socks are "a thing" with teens righ now. Knee socks are more popular than short socks.
Hoodie sweatshirts and cardigan sweaters are popular, and they're so simple that you can't go wrong.
PJs are always nice; and if you're feeling unsure about colors, they're less intimidating because they're only worn in the house.
Fancy lotions and shampoos from Bath & Body Works. I see LOTS of those items friend-gifted among the high schoolers.
Fancy pencils and notepads, a new calendar for the upcoming year, or other "nicer" office supplies. Not stationary; teens don't write letters.
Many teens are very environmentally conscious; a re-usable water bottle would be nice, and that's something people don't mind having in duplicate. Fill it with candy. Likewise, Tervis Tumblers are big favorites, as are travel coffee mugs.
My 17-year old wants college stuff more than anything else right now. She's getting her college laptop and some accessories to go with it, some inexpensive dishware, and . . . seems like something else, but I can't remember. I wouldn't get bedsheets; that's something that's very "visible" and she'd likely prefer to pick that herself. But towels, an alarm clock/ipod charger, a sweatshirt with her college name -- those things'd be welcome. She just bought herself a lap desk a few weeks ago, but that'd be nice for a college-bound student. Or one of those big body-pillows; she could use it now at home, then later take it to college.
For those who insist upon gift cards, choose carefully. Last Christmas my daughter received a Walmart gas card from a well-intentioned but clueless family member. The closest Walmart that sells gas is 30 miles away. I "bought" the card from her, and I still haven't used it. Also, both my girls still have the movie gift cards they received last Christmas; they've been to the movies a couple times, but either they've met friends at the wrong theater or they were someone's guest. They never seem to jump right on spending their gift cards.