There is a couple of ways that I handle this depending on the look needed. 

The image on the left shows the hair with lines and overdraw set to about 70% - it's  a little more of a drawn look. 

The image on the  right is what I do more often. I've engaged the Toon shader function on the actual hair shader in the shader room. The I boosted the shadow highlight settings, set the levels at 4 and darkened the shadows.  - All this is to preserve the transparency look and flatten out the colors but maintain some of the texture maps detail. This method is a great way to individually control the look of texture maps without specifically setting Toon overrides.

Mark