UIBarButtonItem *btn = [[UIBarButtonItem alloc] initWithTitle:@"Custom" style:UIBarButtonItemStyleBordered target:self action:@selector(someSelector)];
self.navigationItem.leftBarButtonItem = btn;
[btn release];

- (void)viewDidLoad {
UIBarButtonItem * backButton = [[UIBarButtonItem alloc]initWithTitle:@"Back" style:UIBarButtonItemStylePlain target:self action:@selector(backView)];
//navBar is your Navigation Bar
navBar.topItem.leftBarButtonItem = backButton;
}
-(IBAction) backView
{
    [self.navigationController popViewControllerAnimated:YES];
}