Hi - Ok, don't hate me, but here's what I think you might do... get out your kneaded eraser, and back off of the shadow areas about halfway... this will flatten the overall hair mass out... then, like Melissa said, go back in with a finer point and put in some dark strands - keep them tight together... finally, go in and pull out highlights.... basically, what you're doing is giving volume to the hair, dimension... note that the clumps of hair cast shadows too...
this is my son's portrait I did a few months ago... his hair was pretty hard to do, seeing that it's very blonde... but I hope this helps...

Email me:
portraitart@ev1.net