Refine the light
Once I feel that I have a good base painting I start to refine the light. When creating light I usually switch between three layer styles, depending on how harsh I want the light effects to be. Those layer styles are Overlay, Soft Light and Hard Light. I do a lot of trial and error, letting my mind tell me when something looks off or not. Reducing the Opacity of the lighting layers works, too. When lighting the scene, I go for two separate layers: the first one is set to Overlay and the other is set to Hard Light. When working with light, I always use a big, Soft brush. Only for direct light would you want to use a Hard-edged brush.