The specific problem of painting hair has stumped me longer than almost any other subject. Here's what finally resolved it.

The wrong approach: painting individual strands. This produces a surface-level busy texture that looks like scribbles at anything but the closest zoom level. It's also enormously time-consuming.

The right approach: painting hair as a series of masses with a clear light source. The mass of hair has a highlight (the area most directly facing the light), a mid-tone body, and a shadow side. Within the mass, I paint large chunk subdivisions — the major sections the hair separates into. Then, only in the most prominent area and only at the finest level, I paint the indication of individual strands.

The proportion: roughly 70% of the time on the mass values and shapes, 20% on chunk subdivision, and 10% on strand indication. Most beginners invert this proportion, spending most of their time on strands and wondering why the hair doesn't read.