Great book on the subject called the Million Dollar Consultant:
http://www.summitconsulting.com/scg5.html
I learned a lot from this way back before the internet was big. Among the words of wisdom are that there is almost NO hourly rate that would be high enough to really create financial independence in the consulting business. When all the down time, free time, vacations, sick time, etc. is figured in, you would almost never get ahead. Rather you should always attempt to do jobs by the project - because if you offer a value your customer will go for it.
Example: If I am an energy consultant, I come into your factory and say “I’ll get things right for $200 an hour” and you might flinch. But If I said “I can lower your monthly energy bill by $3,000 per month with our program for a one time charge of $12,000”, well - heck, you would be happy with a 4 month payback and then savings forevermore.
My business is web publishing and content, but maybe only 1/20 of it is by the hour. The rest is mostly advertising which I run on my site (which is mostly yearly based).....so that is more like a “project”. It might only take one hour of my time to renew an advertisement for $1500. Of course, it takes “free” time to create content and framework etc, but life is much better not billed by the hour.