#7 Using Layer Masks – Photoshop for beginners
More beginner tutorials here: http://www.psdbox.com/category/beginners-serie/
Learn how to work non destructively with layer masks.
Landscape image: http://www.sxc.hu/browse.phtml?f=view&id=1086684
Sky image: http://www.sxc.hu/browse.phtml?f=view&id=108861