Cmd-T : resize current selection
Cmd-I : inverse mask selection
Copy mask to another layer: Cmd-click a mask to select (the mask outline in the layer view). Copy, select other layer, paste.
x – toggles foreground and background colors.
b – select brush
[ ] – increase / decrease brush size
This series of tutorial videos is probably the best I’ve come across so far, very clearly explained.
The best tip ever for layer masks! : “White reveals, Black Conceals”