If you're looking to mitigate the cost of upgrading to Apple's 'iPhone 6' by getting the most cash possible for your current iPhone, buyback service Gazelle has just launched its most aggressive price-lock promotion yet: up to $350 for your old iPhone if you lock in a price quote today (and you don't need to send it in until Oct. 10 — long after the iPhone 6 is expected on the market).

Extended Price Lock

Between now and Sept. 9, existing iPhone owners can lock in a top-tier price quote over at Gazelle.com. And this year the company is upping its price-lock window from the historical 30 days to over 50 days in order to make sure consumers can purchase and set up their new 'iPhone 6' long before having to drop their old iPhone into the mail. Gazelle says anyone who locks in a quote before the Sept. 9 will have until Oct. 10 to send theirs in.

Why now is the Best Time

Trade-in values typically drop 20 percent in the 60 days around the introduction of a new model, with much of that dip transpiring from 20 days before the unveiling to 20 days afterwards, as can be seen in graphic provided by Gazelle (above). Therefore consumers who plan to purchase an 'iPhone 6' right after it's announced and then sell their old iPhone should consider locking in a rate now, when buyback offers are at their peak. There's no downside to locking in an offer now, given that you are not bound to send your device in if you later choose not to, and can always lock in a higher rate later with the same — or alternative service — if a better offer presents itself.

How Much will I Get?

As can be seen in our Trade-In Price Guide that tracks the prices buyback services are willing to pay for used Apple devices, Gazelle this week is offering up to $120 for an iPhone 4S, up to $205 for an iPhone 5, up to $195 for an iPhone 5c, and up to $350 for an iPhone 5s. It's also the buyback service that has thus far proven to be the quickest, most reliable and hassle free for consumers over the past few years, which has helped make it the nation's leading consumer electronics trade-in service with more than 2 million trade-ins processed — and $175 million paid out — to date.

So How Does it Work?

Step 1: To begin the process, identify your current iPhone from Gazelle's website (by selecting your model, carrier and capacity) and then quickly assess its condition — broken, good, or flawless.

(If you have an AT&T iPhone, Gazelle will also present the extra step of asking if you're submitting your phone locked or unlocked. An unlocked phone means more cash, and mostÂ AT&T customers will be eligible to unlock their old phoneÂ after upgrading to 'iPhone 6.' It's easy, here's how.)

Step 2: If you don't already have a Gazelle account to log into, you'll be asked to quickly register by providing an email and password. You can then select you prefer to be paid: by PayPal, check, or in the form of an Amazon Gift Card. Alternatively, you can donate the proceeds from the sale of your iPhone to a charity of choice.

Gazelle will then ask for your address so that it can send you a pre-paid shipping box that you can use to submit your iPhone when you're ready. Alternatively, you can choose to have Gazelle email you a pre-paid shipping label and you can affix it to your own box.

Step 3: Within days, the pre-paid shipping box will show up on your doorstep. Sometime before Oct 10th, simply place your old device in the box and send it off to Gazelle. (You don't need to send in any chords or power plugs, just the device itself.)

Once your device is received, Gazelle will completely wipe the personal data off your phone by returning it to its original factory settings. It will then pay you via Paypal, email (for an Amazon Gift Card), or issue a check for the cash value of the device you locked in today. The offer in recent years has become an increasingly popular way consumers to afford upgrading each year to Apple's latest handset by offsetting the cost of those new devices with the cash they make from selling their old iPhones during peak buyback season.

Still, a study published as recently as this February found that iPhone upgraders as a whole are still leaving nearly $13.5 billion worth of devices in dresser drawers to collect dust.