Instructions:
Texture, texture, texture. Once the flow is done, it's all about the texture. A really, really nice way to accomplish that wild puzzle of strands that makes up a hair is to paint strand after strand after strand on top of each other, then on top of those on a new layer, strand after strand. Do up to five different layers with different colours, set them to different opacity, blur the strands individually either with smudge or with the blur tool and just keep at it.

Varying between a slightly bigger brush and a really small one, you'll trick the eye into seeing small strands even where there aren't any. Remember that one lock that will have super-details? Focus on that one.

Common mistakes:
Thinking that every individual strand has to be painted all over the head. When I say a lot of details are nice, I don't really mean that level of detail.