A red iPhone 7 sitting screen-up on a wooden surface

The value of your current phone is declining fast, so if you're hoping to sell it for as much as possible before you upgrade to your shiny new device, speed is key. Essentially, as soon as Apple announces a new iPhone, the value of any past iPhone begins to plummet. So the closer you get to the release date of the new phone, the less your old phone is worth on sites that allow you to sell your used devices, such as Gazelle, Amazon and Swappa.

Fortunately, Gazelle announced on its website that for a limited time you can lock in the value of your iPhone, as long as you sign up now and have it boxed and mailed in by Oct. 22.

In addition, Gazelle has organized a "Get Paid to Trade" sweepstakes for those selling their devices during the promotional period between now and Oct. 22. If you participate, for each device you sell you get a chance to win the approximated value of the new iPhone 8 on a Visa gift card.

Selling your iPhone with Gazelle is fairly straightforward, so it shouldn't take you much time to hop on this deal - the website guides you through. As a test I went through the steps myself and it took me about three minutes from start to finish. However, if you need help better understanding the device-selling process, we've got you:

How to sell your iPhone