Key to this is realize you have leverage in the transaction. It may not seem like it but you do. Sprint is still on the fence hanging on for dear life. They need you more than you need them, so negiotate for what you want. Be friendly and professional though. Explain to them you are willing/able to leave if you can't come to an agreement.
First things first. Go and try out a bunch of devices and figure out which one you want the most. Then call retentions and try to work a deal. Just don't get testy on the phone with them, you will do better being friendly but explaining to them that you have options with other carriers. They very well may work a deal with you where you pay upfront but they take 20 bucks of your bill for a year for example. Happened to me on the tp2 which I ended up getting for free
I would call retentions though.