I assume you mean without paying an arm and a leg? In a word "you can't" as far as I know. Unless they've fucked up somehow and could be deemed to be in breach of their side of the contract etc.
My 3 contract was due to terminate in December of this year. I called them and said that I wanted to terminate my contract as other providers are offering much better deals and that I don't feel the current contract represents value for money anymore. The guy said he could put me through to disconnection but asked whether I would be willing to speak to an advisor first to see if they could better what I had come across. Of course, I hadn't even bothered to look at other deals as this is a waste of my time. I made up some deals that I had been offered and he had to go one better to keep me. He gave me an offer but I was sure I could get better still. I told him I'd think about it and took the number for his department. I called back the following day and presented a different advisor with the same scenario and was presented with a better deal than the man had made me previously.I'm aware that 3 in-store is quite different from 3 call centers. In store, they don't seem to be able to negotiate deals whereas on the phone, I was practically dictating what I want. Just my experience.
incidentally, this is the only reason i want a pearphone.