How to get top trade-in dollar for your old iPhone while waiting for the new one

You can get up to hundreds of dollars if you follow this advice.

Planning on selling your old iPhone so you can buy the iPhone 6 when it launches in several weeks? Here's how to make sure that you get your top trade-in dollar.

A number of sites will buy your old iPhone, but keep this in mind: The longer you wait to sell, the less money you're going to get. That's because Apple buyers tend to work themselves into a frenzy leading up to launch and after, and start unloading their old devices. The law of supply and demand takes over, and prices drop.

Gazelle, which buys old iPhones and other electronics, told CNN that in past iPhone launches, the price paid for used iPhones dropped 20% in the month before and the month after launch. In fact, Gazelle calls the iPhone launch day "Gazelle Christmas" because of how many people unload their iPhones when the iPhone hysteria hits.

The clock is ticking. Every day you wait to sell is a day in which the price might drop.

Before selling, you should unlock your device. For information about how to do it, check Apple's support information here. Once you decide to sell, your best bet is to shop around, because different trade-in sites offer different prices for your phone. What you'll get for it varies by model number, its condition, how much storage it has, and even by which cell phone provider you use.

Several sites will buy your old iPhone. At uSell, as I write this you'll get $292.50 for an iPhone 5s in good condition with 64 GB of storage from Verizon. At Gazelle, the same model would get you $315. NextWorth will pay $294. will pay $400. And Glyde will pay $413.

That's just for one model though, and only at one point in time. Prices change, so you should price-compare at all five sites for your precise model.

Of course if you sell now, that means you'll be without an iPhone between now and when you buy the iPhone 6. But Gazelle has a way around that problem: You lock in the price now, and only send your old iPhone to Gazelle after you've bought your new one.

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