Many years ago, I did consulting at $125/hour, but if I was going on-site (particular in another city), I'd bill by the day, at $1000/day (often putting in 10 hours a day, so $100/hour) with some sort of minimum in place, so I wouldn't drive or fly somewhere to only be needed for one day. The reason I could charge what I did was because I did some stuff that led to me being well-known. I wrote a book; it didn't sell very well, or pay me much, but when you say, "I wrote a book about system administration, published by No Starch Press" it raises you to the tier of people that can charge $125/hour without hesitation.

Also, when you have a long-running consultancy that specializes in one niche (mine was web caching, website acceleration, etc., though I also did good business in developer infrastructure, like building systems for packaging and testing and revision control, etc.), you'll have fewer customers, but they'll be customers who will value your skills. A jack of all trades, who is actually really fucking good at all trades, is incredibly valuable, but it's difficult to market that because it's difficult to prove you're really good at everything. Proving you're good in one field is just a matter of following the mailing lists and forums in your field, and being talkative and helpful, writing a blog about your subject, and eventually turning all that writing into a book or magazine articles or something.

Of course, I never took it to the next level, as this article discusses. I was still billing by the hour, and still being thought of as a (pretty high end, and difficult to find) commodity. So, I made a decent living, but it didn't scale.