Feeeeeature Creeeeep

http://sixrevisions.com/project-management/eight-tips-on-how-to-manage-feature-creep/

Save yourself a lot of pain in your professional life and learn from the above article.

In my experience (combined with a lot of reading/discussing other artists' own trials and tribulations), learning those lessons the hard way means uncomfortable stretches of unemployment or total abandonment of the industry to avoid dealing with clients.

Hope this prevents some disasters (or at the very lest gives you a cushion to fall on)!