Almost everything we do is fixed-price. I can't think of a client that has screwed us. But we go out of our way to find ways to work ("for free", gasp!) with clients; nothing is more valuable than the relationship.
Guess who they call when they have another job? Me. Guess how much more they're willing to pay than new clients? A lot more.
Customer service being a first priority is not just a marketing phrase - it really does make a huge difference.
That being said, it is very important to define a clear scope, explain how much extra certain pieces would cost if they go out of scope, etc. Most clients are very receptive to changes in the quote - note the use of the word QUOTE - when the scope changes and you explain why it will cost more.
Maybe the trick is, pick good clients.
(The startup was an ISP, in 1996. Even in 1996, an indie ISP was hard to differentiate from the majors and incumbents. Mike and Tracy, the guys who ran the company, managed to build an unassailable reputation for customer service --- to this day, I don't understand how they did it, because I don't see how we did anything different from the other ISPs, other than to tell ourselves we had awesome customer service. But our ISP kicked ass up and down the block based almost entirely on that reputation. I'd love to get some of that mojo at Matasano.)