My trick is layering and backing in stages. First I sculpt the rough shape of the hair directly on the head to get the size and shape right. You might even want to bake that so the rough shape doesn't shift as you sculpt detail. Once you have the rough where you want it you can start layering on thin strips and sculpting it to the desired hair texture. There are lots of different ways to texture but I like to use the bottom of my x-acto knife's handle to "roll" lines into the clay in successive steps. Easier to show than to describe.