Custom back button as navigation item left bar button.

MyViewController.m

#import "MyViewController.h"
#import "UIViewController+JIMBackButton.h"

@interface MyViewController ()

@end

@implementation MyViewController

#pragma mark - UIViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    
    [self jim_setupCustomBackButton];
}

UIViewController+JIMBackButton.h

//
//  UIViewController+JIMBackButton.h
//  CupMaster
//
//  Created by Juan Felipe Alvarez Saldarriaga on 11/7/13.
//  Copyright (c) 2013 CupMaster. All rights reserved.
//

#import <UIKit/UIKit.h>

@interface UIViewController (JIMBackButton)

- (void)jim_setupCustomBackButton;

@end

UIViewController+JIMBackButton.m

//
//  UIViewController+JIMBackButton.m
//  CupMaster
//
//  Created by Juan Felipe Alvarez Saldarriaga on 11/7/13.
//  Copyright (c) 2013 CupMaster. All rights reserved.
//

#import "UIViewController+JIMBackButton.h"

@implementation UIViewController (JIMBackButton)

- (void)jim_setupCustomBackButton
{
    self.navigationItem.hidesBackButton = YES;
    
    if ([self.navigationController.viewControllers count] <= 0) {
        return;
    }
    
    if ([self.navigationController.visibleViewController isEqual:self.navigationController.viewControllers[0]]) {
        return;
    }
    
    self.navigationItem.leftBarButtonItem = [[UIBarButtonItem alloc] initWithTitle:@"⬅︎" style:UIBarButtonItemStylePlain target:self action:@selector(jim_customBackButtonPressed:)];
}

- (IBAction)jim_customBackButtonPressed:(id)sender
{
    [self.navigationController popViewControllerAnimated:YES];
}

@end

Don't return in middle of statements unless it has an advantage of readability, e.g. a big-ass if statement.