You really want to check out a couple of posts over at the Consumerist: "7 Confessions of a Cingular Sales Rep" and "8 Confessions of a Former Verizon Sales Rep". Both are chock-full of tips and tricks for getting the most out of these blood-suckers (the carriers, not the reps) when it comes time to upgrade your phone or change your plan. The gist: use the reps' incentives to your advantage. They get a big boost from text message plans, so offering to sign up for one of those should net you savings elsewhere. And with carriers constantly raising the costs of non-plan text messages, you probably should be getting one of those unlimited plans anyway (and the unlimited data plan too, lest you end up with a $8,677.29 phone bill).
Update: Add confessions from a Sprint rep to the mix.