Most of the time, photographers use artificial sources of light as their main light when taking portraits. To make the best use of this source of artificial light, you should ask your subject to face towards it. However, you cannot implement this strategy when the prime source of light is the sun. Even with the artificial light, you can do this only if it has a soft light. In this case, you can use a strobe or a flash together with accessories (light modifiers) such as umbrella or a soft-box.
By making your subject to face the direction we mentioned above (towards the source of the light), you will be able to make the light to appear softer. In addition to that, you can expect a more flattering light with this strategy. You may not capture any annoying shadows on the face of the subject.
When the subject faces directly towards the source of the light, the face will give some glowing effect (which is natural). This effect will automatically highlight the face of the subject (key object in the shot). However, you must avoid using too harsh light; it will do more harm than good to your photo.
Use a light modifier and make the light source coming from above the eye-level of the person you capture. If you like, you can let the subject face tad towards the left or right of the light source.