As rumors spread that the Apple iPhone 5 will hit shelves as early as next month, now may very well be the best time to sell your iPhone 4S. The phone's trade-in value is still high, but will likely drop as the iPhone 5 release date nears and phone shoppers increasingly stop purchasing existing smartphone models, holding out for the iPhone 5 and its new features. 

Experts maintain that the best course for those who plan to upgrade to the iPhone 5 is for them to go ahead and trade their current phones in as soon as possible, a proposition that may be untenable for some as it would leave them without a phone for at least a month. But if you can figure out a way to get by without your current iPhone for a bit, it may be the most economical way to proceed.

"We always get a rush of people who wait to get a quote until they have the new [iPhone] in their hands," Jeff Trachsel, chief marketing officer for electronics trade-in company NextWorth told CNN Money. "But as the volume increases, the value of your phone declines."

In other words, trade your phone in now and you could get a pretty penny, but first when the release date is officially announced, and again when the phone subsequently hits stores, the payment you can get for your existing older-model iPhone will most likely see a significant drop, according to experts. It's happened time and time before, they say, plus you'll get to avoid the frenzy of getting rid of a phone on or near a launch date, which can be nerve-wracking and time-consuming.

Huffington Post presciently points out that the biggest drop in the amount companies will pay for the iPhone 4S will likely come on the day when a reliable source reports that a solid release date has been announced, as happened on Sept. 21 last year. All Things Digital broke the news that the iPhone 4S would be revealed on Oct. 4. That announcement trumped the announcement date itself and even the release date in its impacts on the price companies were willing to pay for an iPhone 4.

The way many trade-in services like NextWorth work is by offering a bid on on your device that you can choose to take or leave on the table over the next few weeks, after which point the bid will expire. So CNN Money suggests a little bit of a tricky maneuver that can help you get the absolute best deal with the least headache:

"Smart traders play arbitrage: Lock in a price now, then wait until you have the new phone in hand to actually send in your old model," the site suggests.

And you can get some good cash for an iPhone from Nextworth. A 16 GB black iPhone 4S will get you $300, and a 16 GB black iPhone 4 gets you $175. Not bad for something that some people will consider obsolete when Apple's iPhone 5 hits the shelves sometime in the coming months.

Joe McKeown, a marketing vice president at trade-in service ReCellular, told CNN Money that the prices will likely drop quickly:

"Two weeks from now, we'll probably see lower value," he said.

So if you want to go ahead and trade your iPhone in sometime soon, check out the following websites: Gazelle, ReCellular and NextWorth.

Some prognosticators predict that the iPhone 5 release date will be announced during a Sept. 12 Apple event, and some say it could hit stores as soon as Sept. 21. There is no official word yet from Apple, however, so for now all we can do is keep reading all the rumors and wait. And maybe sell our current phones in the meantime.