This portrait of my son was taken a few years ago. The first image is okay. It's from a single flash bounced from the ceiling.
The second image has a lot more sizzle but still uses only one flash.
The difference? Foamcore.
I had Adam hold a piece of white Foamcore under his chin just out of view of the camera. The light from the flash was diffused twice—once when hitting the ceiling and again when bouncing up from the Foamcore. This fills in a lot of shadows under the nose, chin and around the eyes. It gives the whole image more of a bright glow. By comparison, the shot without the fill looks downright gloomy.
Could it be even better? Yes. A second light to produce a catchlight in his eye would have added just a little more life to his face. In post production, a subtle vignette could be added to draw more attention to his face. We're not really interested in his orange shirt.