If you've not read Nathan Fowkes' book "How to Draw Portraits in Charcoal" this is a great general charcoal drawing tip, not *just* for portraits. He suggests sketching in the subject first, lightly, with an orange colored pencil. Then you can start laying value on top of that with the charcoal, like you're painting with it. It's especially helpful if you've got a fairly complex subject! Here's a charcoal Socrates bust I drew awhile back; you can still see a bit of the orange peeking through here and there but for the most part it gets completely covered up and vanishes under the charcoal.