I saw a great drawing exercice on tke Sketch Club's blog (http://sketchclub.blogspot.com/). Here is the description they give;
After lunch, Sketch Club identifies a person of interest, usually for their unique or distinct characteristics. The subject is briefly studied without drawing. Once back from lunch and back at the drawing board, the subject is drawn from memory.
This is exactly what I did with these 2 guys I saw on the street. They had strong features that easily printed in my mind.