Thanks! You have to plan the portrait well and leave the whites white from the start. Not easy I know but the only way to do it (unless you want to 'cheat' with white pastel or paint but I don't think this is necessary). Works for me!
You're tempting me to give charcoal another shot you know :S Do you use sticks or do you prefer the pencils? I have some of both around the place and used to find the sticks much more fun to play around with, your not drawing unless you're coated in charcoal/pastel :P