Trading up iphones - how to do it?

[maggiethecat]

My contract for my iphone 4 has just expired and am thinking of trading up for the 4s. Has anyone done this and about how much did it cost you?

First look up on the internet how much a new customer would have to pay to get a 4S. Then phone up your provider (O2, Orange, whatever) and tell them you are thinking of leaving.