Guy Kawasaki talks about his friend Doug Lawrence about the similarities between public singing and public speaking. I've never been a singer, but I do have to get up before groups and speak occasionally. If you're a missionary you are a speaker. If you are a speaker, you can improve.
Since I drafted this post he's added a second part. Rather than give you my limited insight here, let me just link to the two parts.
Part I: Speaking as a Performing Art