If you want to keep your phone as long as possible: NextWorth.com
The new iPhones will probably be unveiled next week, but they might not be available for sale until Sept. 20. So if you want to sell now but avoid being left stranded without a smartphone, you should go with NextWorth.com.
There, sellers can lock down a quote for their device that will be guaranteed for 30 days. That means you can sell your iPhone for how much it's worth now and keep it until after you get your new device.
The website will send sellers a shipping label that they can use to send off their device within 30 days of locking down their quote.
NextWorth.com will ask sellers whether the device still works and if the screen is cracked. Right now, you can get $340 for a 16 GB iPhone for AT&T.
Depeding on your device and its condition, prices will vary, so make sure you look around before selling your gadget.
Besides the websites above, you may also want to consider trading your device in with Amazon.com, Best Buy, RadioShack, and Gazelle.com. You can also sell on Craigslist or EBay and possibly get more money back, but those websites will require you to be much more active as a seller.