Earlier this summer, one of my regular clients passed off a new client to me-- mostly because they didn't want to work with someone difficult. They warned me in advance, and then added, "Charge them whatever you want." So, I did, and they are now my highest-paying clients.
I try not to work with people who live locally, especially because the few who have contacted me about work at notoriously difficult personalities, and I am not convinced they would ever have paid me. So, I quoted them fees that were so ridiculous, I never heard from them again.
I now recommend to everyone who consults that they develop an Asshole Fee. It will make you laugh, at the very least. You don't necessarily have to list it directly on your invoice either. And I don't feel guilty for charging people extra hours when they are assholes: If I didn't, then I would stew and simmer, and I would make less money in lost productivity. So, I just charge them, and get on with it. Oh, how I wish I could have done this when I worked in an office.
Do you have an Asshole Fee? How did you establish it, and when do you invoke it? And if you don't have one... you should.