Your iPhone 5S in excellent condition could be worth $350, even after the iPhone 6 drops in a few weeks, rendering that current Apple flagship phone instantly less cool. The key to getting that maximum trade-in value for your old iPhone is locking in its value right now.
As of this writing, trade-in site Gazelle is offering up to $350 for a 64GB iPhone 5S, which should get you nearly halfway or better toward the cost of a brand-spanking new, unlocked iPhone 6, if that's what you're after. Gazelle allows you to accept an offer today to lock in the current price, but will give you until October 10 to actually send in your trade-in, giving you plenty of time to get your new iPhone 6 or to disavow the Digital Age and disappear into the wilderness with $350 of camping gear.
Of course, if you take that same used iPhone 5S in excellent condition over to eBay, they'll advise you to start bidding at $288, or just go big and advertise it as a "Buy it Now" item for $720. Indeed, as I was writing this post, I came across just such a used iPhone 5S that managed to fetch $502 in an auction with more than 50 bids placed.
Selling on eBay is certainly less of a sure thing than going with a buyback site like Gazelle, but if you're a prolific Amazon shopper, the mega-retailer might be offering the best deals around. As of Monday, an unlocked iPhone 5S in excellent condition could be traded in for a $400 Amazon gift card, or a $364 card if the iPhone is only in good condition.
There are a handful of other trade-in options, but the three above seem to be offering the highest prices at the moment. But these prices are likely to drop quickly as the iPhone 6 launch approaches and circumstances and offers vary, so be sure to shop around for the best deal for your situation, just be sure not to delay too long. No smartphone wants to end up like my poor water-damaged Droid 2, still sitting under a pile of scrap paper from 2011 in my bottom drawer...