Begin creating products. Speakers are actually in the information business. As you develop your subject, study it, write articles about it, and come to new conclusions, your body of knowledge becomes more valuable, and salable in many other formats. T-shirts, hats, and even toys based on your expertise. Keep adding new products.
Buyers complain about the lack of accessibility to a speaker more than any other issue.
No business will survive for long if their best salesperson, and main decision maker is always away from where and how their clients can contact them. Yet, that is the dilemma for a professional speaker. Also, a 24 hour dedicated fax line that you or your team have immediate access to, phone lines dedicated to your business. It also means a system that allows 30 minute turn around on inquries and requests for materials. Speaking fees are paid according to how good you are as a speaker, how unique, interesting, and informative your material is, and how well known you are.