Instructions:
All right. Flatten. Just flatten all the layers if you haven't already. Chances are you'll have way, way, way too many layers at this point. Then, on top of your neat, nice one-layered image, add a new layer and set it to soft light.

Now you're going to do something that likely looks weird, but it really adds to the effect when viewed from a slight distance. Pick a brush, almost any brush, and make it decently small. Then, pick a colour from the canvas - set it as your background colour and then pick another colour as foreground. Go into brush settings and set the pen pressure to colour, and... scribble. Just scribble over the hair, shifting colours as you go, using a low opacity on the layer, and adding texture and depth to the hair. If you use dark colours, you'll also add a nice amount of contrast that will help the realism along a fair deal.

Common Mistakes:
Just skipping the random texturing. It really helps to add life to hair even if it seems like pure madness, so don't leave it out.