Your current iPhones will fetch the highest prices on the trade-in market if you cash them in before the new iPhone comes out.
That's because millions of other consumers will also be looking to trade in their phones. So that will flood the market and prices will go down.
Most online trade in companies will quote a price for your phone and lock it in usually for 30 days. You can hang onto your phone until the new one comes out and still get the higher price.
For the iPhone4S Gazelle.com is offering $277, Guzu.com is offering $285, and NextWorth.com is offering $300.