Here's another option - I think this only works if you don't require translucency on your nav bar (I didn't). I just added a 1 pixel high UIView to the bottom of the nav bar (1 pixel below the nav bar) with the same colour as my nav bar:
UIView *view = [[UIView alloc] init];
[view setBackgroundColor:self.navigationController.navigationBar.barTintColor];
[self.navigationController.navigationBar addSubview:view];
[view mas_makeConstraints:^(MASConstraintMaker *make) {
make.height.equalTo(@(1.0f));
make.leading.trailing.equalTo(self.navigationController.navigationBar);
make.bottom.equalTo(self.navigationController.navigationBar).offset(1.0f);
}];
I'm adding the constraints using Masonry.