A lot of the work I do in Photoshop involves brushes. Usually for painting in Layers Masks or Adjustment Layers. 2 simple shortcuts that dramatically speed up brush use are: [ and ] to decrease and increase brush size, and Shift [ and Shift ] to change the Hardness. A final quick shortcut is to use Shift + Alt + P (Shift + Option + P on Mac) to toggle the Airbrush.