Custom back button in navigator

[melmoup]

Hi,
I'm tring to define my own BackButton in a Navitagation Bar.
I can do that but I would like the button to be like regular back button generated when pushing a view, so a arrow like button.
The one I get is a standard rectangular button:

Thanks in advance

[self.navigationItem setTitle:@"Testing"];
UIBarButtonItem* testProgBack = [[UIBarButtonItem alloc] initWithTitle:@"Go Back" style:UIBarButtonItemStylePlain target:self action:@selector(flip:)];	
[self.navigationItem setLeftBarButtonItem:testProgBack];