The purists will say that you shouldn't have any public methods on a controller that aren't meant to be exposed via a view or redirect to another action. Assuming that you, like I, have encountered a situation where you need/want to do this, however, here is what you do:

[NonAction]
public void Foo(){ /* Implementation... */ }

The System.Web.Mvc.NonAction attribute will prevent your public method from being accessed via any routes.