I'm not going to say I'm a pro at relationships, but I do know a few things about breaking up with somebody. First, you probably should do it in person; calling is OK, texting is so not OK, and sending a canned message you just punch your name into is the worst possible thing you can do.
BreakupText is a new app on iOS that somehow combines all of these terrible, terrible relationship-ending methods into one package. Basically, it's an app that gives you a message template with spaces that you fill in with who you were seeing, your name, how serious it was, and the reason why you decided to break up (you can choose from "I lost interest," "I found somebody else," or "I was eaten by a bear").
BreakupText costs 99 cents, but if that's too steep of a price for an app that automates your Dear John letter--or if you use something other than iOS--you can use the Web version for free, which accomplishes the same basic task.