Check this out. Cellforcash is a cool site that buys your cell phone from you. Right now, iPhones seem to have one of the best payouts. So if you're going to get the brand new iPhone, you might want to sell your old one to these guys.
check out this link to sell your old iPhones
They also take a bunch of other cell phones, and they pay you a specific amount for each model. I think its better than keeping your old phone around in a drawer or something --as I tend to do.