I work in the cell phone industry, and like an above poster said, the #1 way to get a good deal is to immediately say you want to disconnect your service. Agents in the deactivations departments usually get bonuses for every customer they "save" - and they usually have to hit a certain % of customers saved out of their total calls each month to even be elligible for said bonus (or for some companies, to even get paid).
Because of that % they have to hit, when you call in, you need to sound in a hurry, but not angry - if they think the call will take forever and go nowhere, they'll likely just agree to disconnect you, just to get you off the phone; let's face it, if your call sounds like it will take an hour, they would probably rather just disconnect you, and take 3-4 more calls in the time they may have spent with you.
So call in, say you want to disconnect. When they ask why (and they will) say that another company (do your research - find another company with a deal you like through their retail outlets - not through an obscure website) can give you ____. They will likely try to give you a comparable offer. Hesitate, and say you're not sure. Do not agree to think about it then call in later, insist you want to get it taken care of today. Ask if there's anything else they can give you.
If you play your cards right, and you get somebody who cares about their job even slightly, you can usually get a free low to mid-end phone, free shipping on said phone, and then a free cheap accessory (car charger, headset, leather case).
Don't get mad at the agent, they'll be a lot less likely to help you. Don't keep saying "well this company can give me this, and that company can give me that" - you'll sound like you already made up your mind and are just fishing for a deal.
Most of these departments are able to give the same deals that their website offers new customers, and then have a little bit of credit money on top of that (usually for free shipping on the phone, accessories, or a free month of service). Not all do though, so YMMV. I've worked directly with 2 companies (a major, nation-wide on, and a semi-major local one serving only 14 western/central states), and indirectly with a third (a semi-major national one).
//sorry to thread crap