This video will teach you two methods for adding an artificial sky to a photograph where they sky has been accidentally blown out. Of course getting this right in camera is better and more preferable. But sometimes if you do not have fill flash or a reflector, skies will be sacrificed so that people in the photo are properly exposed. You have to decide if the photo looks fine with the white sky. If not, here are a few ways you can fix it in photoshop.
Win Portraiture, Noiseware, and Real Grain from Imagenomic