There's more to drawing hair than just rendering strands of hair, and making it look shiny. You have to make the hair look like it is on a person's head, not some flat thing sort or lying there.

Remember the term "helmet hair"? Even though you don't really want your portraits to look like they have "helmet hair", you do want them to look like their hair is ON their head, kind of like a helmet.

If the skull is round, so is the shape of the hair - after all, it is covering a round skull. And since skulls (and other round things) have shading to indicate their roundness, so does hair. With more elaborate styles, the "roundness" of the head is less obvious, but it is still there. Your shading should always recognize the structure of the skull.

By looking at the hair like a complete unit (instead of individual hairs) you can more effectively shade it in. When the hair is pulled back (like with this woman and her ponytail) it follows the shape of her skull. And the shading of her hair should always reflect this, if only in a subtle way.

Her ponytail should be treated like it's a cylinder - with a "stripe" of highlight going down the side. Once you get the basic shape and shading of the "cylinder", then go in and add stray hairs, waves, and so forth. But don't forget the basic simple shape that makes up the ponytail.

Since the head and the ponytail area both have a light source coming from one direction, the areas that are away from the light source (in this case, the back of the head, and the bottom of the ponytail) will have a darker shadow. Don't forget to indicate this when you are rendering hair. (Click on thumbnail image to see larger version of picture, with shading areas pointed out.)