Unless you're specifically going for the hard light look, shadows can sometimes be a distraction in portraiture work whose impact you might want to lessen. This helpful tutorial will show you how to convincingly lighten them to help even out the luminosity across your subject's face.
Coming to you from Unmesh Dinda of PiXimperfect, this helpful video gives you an in-depth look at how to convincingly bring up shadow areas on a subject's face. The first thing to note is that this is a fairly involved method, so save it for portraits you're really polishing, but the results are definitely worth it, as it splits the shadows into their three components: hue, saturation, and luminosity. By addressing each component independently, you'll be better able to control and evaluate the result to match it to the surrounding skin. Of course, it's best to light your subject as well as you can so you don't have to spend extra time in post correcting things, but of course, we don't always have that luxury for one reason or another, or sometimes, we decide we want to adjust things after the fact. As with most techniques, aim on the side of being subtle, and of course, don't do this to every shadow, lest you'll end up with a very dimensionless portrait.