Thanks for the comments. I guess the trick is don't be afraid to use lots of contrast and when approaching the highlights in pastels don't colour all the hair using the base colour. Make sure you leave areas of paper untouched for highlights for the very light colour (I used white pastel), then do some bold 'flicks' into the darker colour starting from the uncovered paper. Then do some 'flicks' of the darker colour into the new light colour in the opposite direction. Try playing and experimenting without actually drawing any portraits just to get the effect. Not sure if any of that made sense, but I tend to practise an effect I want on lots of plain pieces of paper and scraps with the firm knowledge that this will not turn into a picture before I try to incorporate the effect. On the collar bones and blending I used to put the main colours down and then blend with my fingers, but you need to make sure you're not eating any doughnuts!