When a client chooses a person for a portrait that you're not that interested in, it can show in the finished product. I was not excited about drawing Audrey Hepburn and it showed. I got the mechanics of her face, but was unable to add life and personality. Be careful when accepting a subject matter that you feel inadequate to draw realistically.
Study your subject. Is she sweet? Demure? Or is there a flash of independence that you need to capture. Notice the vague crease marks in her forehead as she lifts her eyebrow. The lips are pursed. The eyes have a steady stare like a cat upon a mouse. You can create personality with your pencil!
When creating a graphite portrait, age is your friend! Never forget to enhance your subject matter by casting shadows on veins, creases and shadows. This brings out their personality as well as making the portrait come alive. (Double chins are a 'no no')
A favorite of mine, I have drawn John Wayne so many times I rarely have to use a photo for reference. RELATE to your subject. Concentrate on the obvious features, then allow your eyes to go out of focus and you will be amazed at the features that are suddenly grabbing your attention.
Even the unblemished and unlined skin of a child can be enhanced using light and shadow. Blond highlights are created by contrasting between dark and a sudden light area on the scalp, creating the impression of blond reflections. Create the glow of youth by erasing an area below the iris of the eye, extending onto the lower lid.
Eyes are indeed the window to the soul and, as such, never deaden an eye by forgetting to add a spark of life through the use of your eraser!! I erased the lower part of his iris and the upper part of his eyelid. It suddenly looks as if there is a light of intensity bringing your subject alive.