If you’re into nondestructive photo editing in Photoshop (and you should be!), here’s the oft-forgotten shortcut for getting that 50% grey-filled layer you’ll need.
Pressing Command-Shift-N (PC: Ctrl-Shift-N) creates a new blank layer any time you don’t want to use the icon in the Layers panel. BUT, if you switch the blend mode in the dialog to Soft Light (which you’ll need to do anyway to make this layer work for dodging and burning), you will then be able to check the “Fill with Soft-Light-neutral color (50% gray)” box at the bottom. Click OK. Now you’re all set with fewer clicks.
Don’t forget, painting with white on that gray-filled layer gets you lighter tones, painting with black creates darker tones—great for contouring skin in portraits to create drama, as shown below.