Carving hair gives your carving life. When done correctly, the workmanship is beautiful and your carving seems complete. To be able to carve hair, you must have a complete visual of what you want your carving to look like and a still, skillful hand to gracefully work the tools. The main thing to remember is that with any new technique, practice makes perfect.
Take your preferred tool and make long S-cuts. Make sure your movements are in a sweeping manner and avoid making straight lines. S-cuts will give the look of realistic movement to wood hair and provide more definition. To make sure the curves in the S-cuts are severe, outline the S-cuts with a pencil on the wood before making the cuts.
Merge every other cut. While the first few S-cuts will be adjacent, every other cut should merge into the last cut at some point. This will add more character to the carving. Just be careful to keep it neat and not to merge too many of the cuts; you don't want the carving to become messy and sloppy looking.
Fill in the hair carving by adding more S-cuts in between cuts. If your cuts are kind of spaced far away from each other, add more cuts in the middle of them to define the carving even more.
Deepen some of the areas. After most of the hair has been carved and intersecting is visible on some, go back over the rest and deepen them even more. You want this to be random and to avoid having a distinct pattern.