Update: Gazelle has announced an extended 50-day price lock window. That will give you until October 15th to send your iPhone in, but you’ll have to lock in your price before Apple’s September 10th event to take advantage.

With the unveiling of a next-generation iPhone expected on September 10th, it’s once again time to start thinking about trade-in options for your current device. While iPhones typically hold their resale value much better than competing devices, previous iPhone models historically experience a big drop in trade-in value in the 3-4 weeks leading up to an iPhone announcement. That’s highlighted in the chart above showing average decline in selling prices for previous gen iPhones leading up to the iPhone 5 launch.

With reliable reports of the next-gen iPhone coming early next month (no word on if that means the rumored iPhone 5S, 5C, or both), now is the time to get the most for your iPhone if you plan on upgrading when Apple’s new device finally becomes available. Apple is testing its own trade-in program that should launch in the coming months, but until then we’ve put together a chart showing where to get the most out of your device today:

You can also get an extra $10 through Gazelle by clicking here, and an additional 10% more at NextWorth here.

iPhone 5:

iPhone 4S:

Some of the carriers offer their own trade-in programs, but they don’t currently offer quite as much money as the services we’ve compared today and sometimes only offer credit towards a new device. You can also take your shot at selling your iPhone on eBay, which could get you slightly more than the services above but without the convenience and 30 day price lock window. If you have a mint condition iPhone (one that looks brand new in the box), you can also squeeze a little bit more out of mobixa.com than the other guys.

NextWorth’s CMO Jeff Trachsel noted that previous generation models will see “a predictable 15-20 percent value decline” in the coming weeks, so you’ll definitely want to start the process now if you’re planning on upgrading. NextWorth is currently offering a 30 day window (up from the usual 21), allowing customers to lock in peak pricing and keep their device for 30 days. Gazelle also offers the 30 day 50 day window for quotes.

We’ll revisit these charts in a few weeks to see just how much the trade-in value of the current iPhone lineup drops in the weeks before the announcement.