‘<’ or ‘>’ moves to the “previous” or “next” brush in the list on the Brushes Panel. ‘<’ or ‘>’ + Shift goes to the first or last brush in list. (At first glance these shortcuts may appear to be making the brush larger and smaller, but that would be because you’re moving from a smaller to larger brush (or vice-versa) in the Brushes panel).
Holding the Shift key will constrain the painting tools to a straight line. In addition, To have a painting tool connect (draw a line) between two points, click once to start a line, hold down the Shift key and click again to set the end point.
To access the Brushes preset picker while anywhere in the image area, control (Mac)/ right mouse (Win) -click with a painting tool selected.
To customize the display of the Brush, select Preferences/Cursors. Select one of the following: Standard (the small iconic cursors), Precise (cross hairs), normal Brush Tip (size represents pixels to be painted with greater than 50% effect), Full Size Brush Tip (size represents all pixels to be painted). Choose to “Show Cross hairs in Brush Tip” if desired. To display precise cross hairs for brushes, use the “caps lock” key.
When the airbrush attribute is in use, tapping a numeric key will set the Flow amount. (If the airbrush attribute is not applied, then the Opacity is effected.) Use the Shift + numeric key to target the other field. Option (Mac) / Alt (Win) + Shift + P toggles the airbrush attribute on and off.
To change the opacity of a painting tool, use the number keys on the keyboard. Tapping one number assigns the percentage of the hit number (1 = 10%, 2= 20% etc. and 0 = 100%). Hitting two numbers quickly will give you that exact amount (5 + 4 = 54%). Note: If you have a tool selected that is not a painting tool, these shortcuts will affect the Opacity on the Layers panel.
I have been asked so many times “What is the difference between the Opacity and Flow?” that although it’s not technically a shortcut, I would like to clarify - the Opacity controls the opacity of the paint (is it an opaque metallic or a transparent varnish?). The Flow option controls the speed at which paint is laid down (are you pressing the nozzle of the can of spray paint just a little, or all the way down?).
If you’re tired of tapping the left and right brackets to increase / decrease your brush size, try using the new Drag-resize brush cursor keyboard shortcut in Photoshop CS4 to make rapid changes to the brush size. Ctrl + Option (Mac) / alt + right click (win) -drag will increase/decrease brush size while displaying a red overlay of the density and feather of the brush for visual reference. Add the Cmd (Mac) /shift (Win) to change hardness. Note: the preview color can be controlled in Preferences / Cursors / Brush Preview).