When to fire a client?

So we've all had a high demanding small revenue producing client. Right? The one that has the potential for large revenue but they are high time consuming, low revenue producing and want everything yesterday. How do you handle this type of client? How do you take them and move them into a manageable revenue producing state? Or when do you have the talk... It's not you, it's me. I'm interested to hear people's thoughts on this.