Apple will officially release the iPhone 5S and iPhone 5C on 20 September, so naturally trade-ins are at the top of everyone's mind.

It's the ideal time to exchange those ancient iPhones for some money to put toward Apple’s upcoming smartphones. Especially for US consumers. Many are locked into carrier contracts and ineligible to upgrade, so they're facing unsubsidised prices if they want the latest and greatest from Apple.

Pocket-lint has therefore researched all the reliable places and websites to help US consumers trade-in their old iPhones for cash or credit - up to $350 worth in some cases. That may not be enough to buy Apple's newest high-end smartphone, but it will definitely pay for the iPhone 5C or get you close to owning the iPhone 5S.

So, read on to find out where you can get the most dough for your gadgets.

1. Apple launched its iPhone Reuse and Recycle programme on 30 August at select US Apple Stores, but it won't roll out to all Stores until early September. Apple limits trade-ins to work as a discount toward a new iPhone bought on contract. The company determines the value of a trade-in based on the condition of the old phone. For example, an unlocked, undamaged iPhone 5 16GB is worth around $279, while an AT&T 16GB iPhone 5 is worth $255.

2. Walmart will launch its trade-in programme in stores on 21 September, and consumers will have the opportunity to trade-in their old iPhone to receive immediate credit toward the purchase of a new smartphone. The retailer will offer store credit ranging from $50 to $300 for more than 100 working, non-damaged smartphones. An example provided by Walmart included $300 for an Apple iPhone 5.

3. Amazon.com offers decent trade-in values for the iPhone 4S and iPhone 5 under its Trade-In Programme, but it only offers cash back in the form of an Amazon.com gift card. Amazon.com gift cards cannot be used on AmazonWireless, but it gives extra money for undervalued iPhones after an official evaluation. Amazon currently values an AT&T iPhone 4S 16GB at $240 and a Verizon iPhone 5 16GB at $350.

4. Gazelle.com gives consumers the opportunity to either trade or sell their old iPhones, and it is currently locking iPhone values until 15 October. The website also offers consumers multiple ways to get their money, such as through a check, PayPal or Amazon.com gift card. An AT&T iPhone 5 16GB in flawless condition is worth $325 on Gazelle, and a Verizon iPhone 4S 16GB is worth $170. 

5. NextWorth.com is like other trade-in sites — it gives a quote, ships a device for free and then pays. Consumers can get their money via PayPal or Gift Card, but NextWorth won't issue payments until the next business day following inspection. It currently values a Sprint iPhone 4S 16GB at $170 and a AT&T iPhone 4S 16GB at $320. 

6. Glyde.com is not a typical trade-in programme; it allows consumers to buy and sell used electronics and provides them with “unique insight into the resale market for used electronics" like which carrier models sell for higher prices. It will also give pricing for old iPhone models with trend and geographical data. The website claims sellers can get up to $380 for an iPhone 5, $251 for an iPhone 4S and $145 for an iPhone 4.

7. GameStop requires that consumers visit a physical store to trade an iPhone. A store associate will inspect the iPhone and then offer cash or store credit for an exchange. Fortunately, through this method, shipping is no longer a hassle. Quotes start at $450 for an AT&T iPhone 5 16GB. But that's just in-store credit. Cash value is $322. 

8. Best Buy.com is a bare-bones way of tradng an iPhone. Just select the model, choose its quality and add it to the cart. Values vary according to condition, accessories and other factors, but a “perfect” Verizon iPhone 5 16GB is currently fetching $315. A perfect iPhone 4S 16GB from the same carrier is $199. 

9. RadioShack.com offers in-store and online options for trading an iPhone, but all transactions are subject to applicable processing fees and must include chargers, cables or other accessories. The iPhone 5 16GB currently gets as much $250, while the iPhone 4S 16GB gets as much as $150. 

10. Verizon Wireless' Trade-In Programme accepts used iPhones from any carrier. It asks consumers to determine the value of their iPhone, and then they receive a Verizon Wireless electronic gift card via e-mail. The carrier values an AT&T iPhone 5 16GB at $300. That's the same price for the Verizon model, too.

Finally, if none of the above offer what you're looking for, you can always try your luck on eBay.com.