Music Composers Unite!
I also have a small passion learning about marketing and copywriting. I listen to a lot of podcasts, and the other day, I listened to an interview of Steven Pressfield, the author of Gates of Fire (awesome book by the way) and The Legend of Bagger Vance.
In the podcast, he talks about the difference between an amateur and a professional.
Mainly, its that a professional knows that his work is never done. He does a project, in his case a book, gets deeply involved with research, but when it is over, he moves on to the next one.
I recommend listening to it. It is really applicable to any profession, not just writing.