The best time to sell your current iPhone is now. In simple terms, the earlier you can sell, the more you can expect to make. The value of older iPhones will decrease immediately when the iPhone 6 is released, and it will keep on dropping as more people sell or trade-in their current iPhone in order to upgrade.
After the newest iPhone is released, the longer you wait to sell/trade-in your old iPhone, the less money you’ll likely receive for it. A lot of sites like Amazon and Gazelle give you 30 days to ship your iPhone to them. As long as they receive the phone within those 30 days, you're guaranteed the rate they quote. However, that doesn't mean you MUST sell to them. For instance, you can lock in a rate with Gazelle now and mail your phone to them after the iPhone 6 is announced. That way you can see if the iPhone 6 is worth it or you can shop around to see if other retailers are offering more money. Gazelle also tends to e-mail coupons to its subscribers -- these coupons basically give you an extra $10 or $20 when you sell your iPhone to them. Bottom line -- there's no rush. Shop around to see who offers the best rate, lock in your rate, and then after Apple's announcement -- you can send them your phone.
If you sell your current phone, your best chance of getting a good price is to sell directly to another buyer through a website like eBay, Craigslist, or Amazon. You can set your own price and the averages tend to be higher. However, this is also the most hassle for you, because you’ll have to list the phone, wait for offers and then package and mail it. Another option is to sell directly to a company like Gazelle, uSell, and Blazing Electronics. There are lots of companies out there looking to buy your old iPhone, and the prices they offer change quickly. The advantage with most is that they’ll give you a quote online after a few questions, provide a free postage pack, and pay fairly quickly upon receipt of the device. You can also trade in your old iPhone. Your last option is probably the easiest, but it’s not going to get you as much money back and it comes with limitations.
If you plan on buying the new iPhone 6 -- make sure you back up your old iPhone so all your videos/settings are saved and can be easily transferred to your new iPhone.