TIP #1: Create variation with warm and cool skin tones. I use the temperature of the colour to vary the surface in an interesting way to communicate the light source.
Tip #2: Save your tonal best for last. You must preserve your tones and hold back from using the top of your range. This way when you final apply the light, it leaps from the surface like dynamite!
Tip #3: Light against dark and dark against light. By increasing the contrast in the painting we can isolate and bring shapes forward from their surroundings. This is especially significant when we contrast the sitter from the background.