The Nose Knows. It should become apparent that I really like having my supplementary light source "on a stick". This gives me some flexibility on the placement of the light in relationship to my subject. You can position the flash to the left or right, or above or below your subject, whatever the situation may call for. But whether your light source is artificial or natural, you can establish a good foundation for effective lighting by aligning the light with your subject's nose.
This lead shot was taken early in the morning during Carnaval in 2008. From the shadows on the street, you can see that the light was coming from camera right, almost perpendicular to the line of sight. Had she been facing me, half the face would have been in shadow and half would be fully illuminated. But by waiting for her to turn into the sun, I got full illumination on the front of her face, accentuating her strong facial features.
A lucky accident occurred when the sun did double duty for this shot. First, it served as a strong accent light that separated the line of his back from the walls of the building in the background. However, light bouncing off a light colored building across the street gently lit the front of his face, separating it from the background and giving the front of the face a three-dimensional effect. The light is so subtle that you might not believe it was really there until you examine the folds of the front of his red shirt. You'll see that the edge is just a bit brighter from the left side, exactly where our trumpeter is facing.
This final image demonstrates that the "nose knows" even at high noon. You can see from the short shadow on the street that the sun is very high in the sky, and while the puppeteer is facing away from the light, his charge is looking directly into the sun. Again, his nose is pointing directly into the main light source. The strong shadows beneath the chin and nose clearly define the face.
So remember that whenever possible, have your main light illuminate our subject's face straight on. As I said, the Nose Knows!