Quote - Aprilgem has the ticket here. It's the fastest and most simplest way of achieving this small triumph.

 

Ironically enough, the technique I actually use the most myself in all of my work is the one described by Rayraz, as I like keeping masks or alpha channels linked with the layer itself. But it was easier for me to describe the channel palette way. ;-)

But everyone's way works just fine. I love that Photoshop provides so many ways to do the same thing. :)