Painting Layers of Hair in Oil Paints

So, I've blocked in the darks, and the midtones with a round No. 6 brush.  While the paint is still wet, use just white paint for the highlights.  They will blend into the wet paint, so that in the shadow areas they will be just visible, which is what we are after.  The whitest whites will be done at the end when the paint is dry. 

I tend to go back into the darks again, as they can become too light when you start feathering off.  The more contrast there is between light and dark, the more depth you create in the painting.