How does one negotiate a better cell phone upgrade with T-Mobile?

I've been with the same cell phone company for 15 years (before they were T-Mobile). I've endured bad service and only gotten four phones over the years. Now I understand that they have a charge for upgrading and then I'm only getting the same price as a new customer. How do I go about negotiating a better deal? Is it better to do it over the phone with customer service, by letter, or in a store? Exact language to use would help a great deal.