UINavigationController "back button" custom text?

You can set the text in the Interface Builder:

Select the navigation item of the ViewController that the back button would return to:

In the utilities panel attribute inspector, enter your label for the Back Button:

I would prefer this approach over setting the title in code as in the accepted answer.

Also note, you need to do this in the view controller one level up the stack. In other words, don't do this in the visible view controller, but in the view controller that you'd see if you hit the back button.
--Tyler

I use this:

// In the current view controller, not the one that is one level up in the stack
- (void)viewDidLoad {
    [super viewDidLoad];
    self.navigationController.navigationBar.backItem.title = @"Custom text";
}

From this link:

self.navigationItem.backBarButtonItem =
   [[UIBarButtonItem alloc] initWithTitle:@"Custom Title"
            style:UIBarButtonItemStylePlain
           target:nil
           action:nil];

As Tyler said in the comments:

don't do this in the visible view controller, but in the view controller that you'd see if you hit the back button