Hair is what you make of it ie a couple of different colours for highlights shadows and midtones.
When succesfully done it looks like hair but in reality its just shapes in the right direction with the odd lines thrown in to emphasize strands.
Here is a technique I tried out (and I tried many techniques believe me)
http://www.wetcanvas.com/forums/showthread.php?t=530816
In the end I just resorted to shapes and defining it as went...In this particular painting I first laid in the highlights with Naples Yellow and then used Van Dyke vrown to gently brush over the NY so as not to blend the brown into the NY too much and once satisfied I then blocked in the darker mass.
