Sometimes it's useful to see what your Photoshop image looks like before you apply a layer mask.
To do that, just hold down the Shift key and click the layer mask
thumbnail itself. You'll see a big X through the mask thumbnail in the
Layers palette, and the effects of the mask will actually not be seen on
your canvas anymore—but your mask is still there. To get things back
to normal, just Shift-click the layer mask thumbnail in the Layers
palette again and everything will be back the way it was.
(For more on Photoshop and layers, be sure to check out my latest book, Layers, The Complete Guide to Photoshop's Most Powerful Feature.