This is an excellent series of videos by Lyssa Adkins:
Part One of Two: http://www.youtube.com/watch?v=TvYqhYEaqMs
Part Two of Two: http://www.youtube.com/watch?v=L9tSjpqeBa4
I highly recommend taking the twenty minutes to watch these two videos. Anyone who is a ScrumMaster, a Project Manager or an Agile Coach should take the time!