Rémi Collin shares a cool approach on where to place code that doesn't really belong in a controller. He creates small, reusable, testable, decoratable classes, called Actions.
Using this approach can seems a lot of classes at first. And, of course the user registration is a simple example aimed to keep the reading short and clear. Real value starts to become clear once the complexity starts growing, because you know your code is in one place, and the boundaries are clearly defined.