Rob worked on umber pastel paper and spends time early on getting the angles as accurate as possible, thereafter staying faithful to the angles and the central line. He notes that values [accurately observed light and dark] are much more important than colour. 
As regards his various types of pastels [technical details can be found at the end of this page] he notes that a set of just 6 tubes of oil is enough to get going whereas it is near impossible to blend pastel pigments. He likes to add pastel pigments without rubbing till late on, using thick pastel sticks for larger areas [Unison are excellent and come in hundreds of shades] and thinner sticks for details such as in the eyes.

He feels that hyper-realism, while technically impressive, doesn't convey as much of an emotional response. Working a portrait too far risks losing that elusive charm inherent in a sketch.

A useful tip, he suggests, is to use a mirror from time to time in order to see the picture afresh, the better to spot what needs further work. You can't beat a fresh eye.

Rob dislikes fixative, observing that it both darkens and dulls a picture. He prefers to put a pastel portrait behind glass to keep it fresh.