Change font of back navigation bar button

To change the appearance of the text in all UIBarButtonItems appearing in all UINavigationBars, do the following in application:didFinishLaunchingWithOptions:

[[UIBarButtonItem appearanceWhenContainedIn:[UINavigationBar class], nil] setTitleTextAttributes:    @{UITextAttributeTextColor:[UIColor blackColor],     UITextAttributeTextShadowOffset:[NSValue valueWithUIOffset:UIOffsetMake(0, 1)],     UITextAttributeTextShadowColor:[UIColor whiteColor],     UITextAttributeFont:[UIFont boldSystemFontOfSize:12.0]    }     forState:UIControlStateNormal];

UPDATE: iOS7 friendly version

NSShadow *shadow = [[NSShadow alloc] init];shadow.shadowOffset = CGSizeMake(0.0, 1.0);shadow.shadowColor = [UIColor whiteColor];[[UIBarButtonItem appearanceWhenContainedIn:[UINavigationBar class], nil] setTitleTextAttributes: @{NSForegroundColorAttributeName:[UIColor blackColor],   NSShadowAttributeName:shadow,   NSFontAttributeName:[UIFont boldSystemFontOfSize:12.0]   } forState:UIControlStateNormal];

Swift:

NOTE: this changes ALL instances of UIBarButtonItem, not just those contained within a UINavigationBar

UIBarButtonItem.appearance()               .setTitleTextAttributes([NSFontAttributeName : ExamplesDefaults.fontWithSize(22)],                                        forState: UIControlState.Normal)

Swift3:

UIBarButtonItem.appearance()     .setTitleTextAttributes([NSFontAttributeName: UIFont(name: "FontName-Regular", size: 14.0)!],                              for: .normal) 

For anyone that did not fully got this to work, here is how i did it, including popped back to the Root ViewController in IOS7:

UIBarButtonItem *backBtn =[[UIBarButtonItem alloc]initWithTitle:@"Back" style:UIBarButtonItemStyleDone target:self action:@selector(popToRoot:)];backBtn.title = @"Back";[backBtn setTitleTextAttributes:[NSDictionary dictionaryWithObjectsAndKeys:                                 [UIFont fontWithName:@"Chalkduster" size:15], NSFontAttributeName,                                 [UIColor yellowColor], NSForegroundColorAttributeName,                                 nil]                       forState:UIControlStateNormal];self.navigationItem.leftBarButtonItem=backBtn;

popToRoot ViewController:

- (IBAction)popToRoot:(UIBarButtonItem*)sender {[self.navigationController popToRootViewControllerAnimated:YES];}

Maybe someone may have use of this.

Swift version of the all mentioned above (excerpt from the original answer) :

let customFont = UIFont(name: "customFontName", size: 17.0)!UIBarButtonItem.appearance().setTitleTextAttributes([NSFontAttributeName: customFont], forState: .normal)

More goodies:
https://stackoverflow.com/a/28347428/469614