Deke McClelland made a screencast for Scott Kelby about masking using channels. I just finished Scott Kelby’s book on Photoshop channels. Heed this warning: this 30 minute training video is not for the light-hearted. It’s not for an intro to Photoshop. If you have some decent Photoshop chops, there are a few gems that Deke covers.
btw, if you don’t know who Scott and Deke are, go spend a day on their web pages.