Like I posted sometime earlier on the iPhone's resale value compared to other smartphones, here is a recent article on where to sell your old iPhone, just before your iPhone's resale price starts falling down, once the new iPhones get unveiled on Sep 10th. While eBay and craigslist might be your best bet if you want the most dollars for your old iPhone, they are the most risky and might need more effort than the other options. Online resale sites like gazelle ($320 for iPhone 5 16GB AT&T) offer an easy solution for mailing your old iPhone for free, while Apple's new resale program (the easiest of the lot ($336 for iPhone 5 16GB AT&T) beats Gazelle if your old iPhone is in excellent condition.
I recently got a quote for my iPhone 5 (32GB) from Gazelle for around $340 and the price is locked till Oct 15th. So I can sell my iPhone 5 for the higher resale price (than when it falls after the new iPhone announcement), if I choose to, after evaluating the new iPhone's features. So even if you are remotely thinking of upgrading to the new iPhone, make sure you start researching the resale options for your current iPhone model.