In the security audit space, I'm not aware of other, more advanced credentials that top the CISA. You might want to consider picking up some other more main-line security certs as well, such as Security+ and CISSP. If you have specific questions about security or audit certification I'll be glad to try to answer them for you. As for getting started as a self-employed consultant, do some research and reading to get yourself ready. Useful titles abound, including: Start Your Own Business (Publisher: Entrepreneur Media Inc.) and Secrets of Consulting: A Guide to Giving and Getting Advice Successfully (Publisher: Dorset House).
This was first published in October 2003