This is an interesting video tutorial that demostrates how how to use a technique called Dodge & Burn, made by the excellent photographer Elena Jasic. It's not the only way to do it, as the mostrly common technique is to create a single layer filled with 50% gray in overlay mode, and then use the Dodge & Burn brushes to enhances the shadows and the lights, but in my opinion her technique has a better control with the two differnet layers.