Here's a few Layer Mask shortcuts:
Hold down Option (PC: Alt) as you click on the layer mask icon to add the layer mask in the opposite of what's selected, or filled with black if nothing is selected.
Once the layer mask has been added,
Shift-click on the layer mask thumbnail to hide it (and then show it again)
Option-click (PC: Alt-click) on the layer mask thumbnail to view the mask
Option-Shift-click (PC: Alt--shift-click) on the layer mask thumbnail to view it as a "quickmask"