Establishing very light and very dark areas in your portraits helps the face to pop. The key is to use small amounts of each, letting the midtones do the bulk of the work defining the face. It’s easy to get carried away with the dark-dark areas, so remember a little goes a long way.
The diagram below shows the key areas that should be highlighted or shadowed. This is a basic face that has light hitting it straight on. As you continue to draw, you can play with your light source, having it hit from different directions. Having a basic understanding of where highlights and deep shadows occur on the face will help you draw from your imagination when no model or photo is present.