Adding lighting, refining shapes
I like to add light early on. To do so, I use the Dodge tool with a Soft brush. Just paint with it where you want to add light, and Alt-click to add shadows. I usually leave the tool set to Midtones and keep Protect Tones checked. Try not to increase the Exposure too much, or you might burn your image. I usually use this trick early, as the tool can burn an image easily.