Door: kelbytv.com/photoshopkillertips
This tip will show you how to copy a layer mask from one layer to another and how to invert the values of that mask.
http://kelbytv.com/photoshopkillertips/2010/02/26/copy-and-invert-a-layer-mask/?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed%3A+PhotoshopKillerTips+%28Photoshop+Killer+Tips+%28Video%29%29