Lighting in passes
This is more of a quick demonstration of a useful technique than a full-blown tutorial, but in some situations it is a very useful way of working.
For instance, for the devil image which kicks off the Monsterosity series, I knew I wanted the character to be backlit by the flames, but still readable rather than silhouetted. So, borrowing an idea from 3D i painted each light source on its own layer. This allowed me to control each source individually and adjust their relative strengths to each other. It also allowed me to independently adjust the colour of each light if I so wished (and I did, to make the fill much redder than I had originally)
I began by painting the ambient pass on to the character's head. I imagined this to be a soft light source coming from below (maybe reflected off the floor). I also wanted to have a sense of the light glowing through his skin, so the values aren't all that dark.
Next i painted in the fill light, this would be coming from some flames on the left, and is the main light used to define his features. It's a harder light, with more defined edges which help outline the form.
The sense of light glowing through the devil's skin really is there now, but maybe a little too strong, so next I painted a shadow pass to define some areas of deeper shade.
Finally I added a rim light which would be coming from the flames directly behind him and thus be the strongest light in the scene. It really helps separate the character from the background.