Here's what I did up to now.
1: First step is I got myself a good reference image (or two, as in this case). Then, using a soft brush, I painted in the basic shadows (100% opacity) and highlights (between 50 and 70%). I like to start with the darkest colour in my pallet first.
2: Still using a soft brush I build up a little higher through the values (35-50% opacity) and try to make some interesting shapes. I use the reference quite a lot for this, picking areas I like the look of.
3: Switching to a hard edged brush (standard round in PS) I pick a high value colour and start to pick out the edges. The method I use is to start with a big brush at a low opacity and then to work down in brush sizes and up in opacity. I start at about 20% opacity. I want to keep to the same basic shapes that I made in the first two stages and not get too messy with the strands of hair. This helps me to highlight the volume that is making up the shapes in the hair.
4: A very important thing I try to remember is to work back into the hair with the dark shadow colour I started with. I keep the opacity at 20, but change the colour from a highlight to the shadow colour and then work under edges that I made in step 3. At this stage I like to get a bit more messy and let my brush cross the shapes I made in step 2 and criss cross the rest of the hair.
5: Another thing I tried to remember was the flow and form of the hair. So I went back over it with a soft brush and re-highlighted the contours. By painting broad lateral bands I think it unifies the hair and stops it looking like painted plastic!
6: I then go bonkers with a small brush (but not 1 pixel if I can help it! Usually 5 pixels is small enough) and a high opacity (40%) and scribble all over it! I love to add loose strands and imperfections at this stage. Then I used a trick! I copy a section of the hair that I am currently happy with, copy it, flip it, place it over some other part of the hair and turn it until I like the way it looks, set the layer to overlay (or screen, if I want a highlight) and then using a soft eraser I blend it into the image.
Each stage inly takes a few minutes. This is the method I use most of the time. I think it works better with curly hair than with straight too.
Hope my explanation was understandable!