Change the Fill Opacity on the Fly
You probably know that (with a non-painting tool selected) you can press a number to change the opacity of a layer (1 for 10%, 2 for 20%, etc.). But what if you want to change the Fill opacity rather than the opacity? Add in the Shift key, pressing Shift-1 for 10%, Shift-2 for 20%, etc.
It doesn’t work on my CS5.