All carriers have retention departments. You are unlikely to get to them by email, so you're going to get on the phone. The key is to recognize what you are worth to them. Most folks have a somewhat exaggerated view of what kind of profit the company makes off of a particular customer. And it's somewhat relative. For instance, AT&T and Verizon might charge more for comparable plans than US Cellular, but they invest a lot more in their network each year. So if a customer is paying $39.99 a month and asking for a deal to keep them from leaving, there isn't going to be a lot of margin to work with. Same goes for 3 phones sharing a $79.98 plan. Generally, the higher your monthly access each month, the more motivated they are going to be to cut a deal to keep you. Best plan is to get to the retention department, tell them that you've had some cellular companies suggesting they have some nifty promotional offers and you were thinking about them, but you were kinda looking for a reason to maintain your long term relationship with your current carrier. Let them suggest an offer and you might come up with something nice.
Of course, this does not apply if they can look at your account history and see that you've called in for hundreds of dollars in credits or exchanged 14 phones. Then you'll be hearing the sad tones of, "We're sorry to see you go. Good luck with your new service provider."