Setting status bar style globally:

In Info.plist set 'View controller-based status bar appearance' as NO
In AppDelegate add
[[UIApplication sharedApplication] setStatusBarStyle:UIStatusBarStyleLightContent];
to
- (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions {
   ......
   ...
}
This solution works for iOS 7.

For cases when you want to set the style differently in each view controller:

If you have an embedded navigation controller created via Interface Builder, be sure to set the following in a class that manages your navigation controller:
-(UIStatusBarStyle)preferredStatusBarStyle{ 
    return UIStatusBarStyleLightContent; 
}