In fact, in a way, requirement elicitation is the most difficult job in project management. It requires the skill of a clinical psychologist ...
When you think about it, most people's internal (that is, imaginary) ideas of reality appear more real to them than reality itself, and the only corrective is pain -- like Ben Franklin said, "Experience is a harsh schoolmistress, but she's the only one fools will learn from."
What's helped me most in that regard is a book from another discipline entirely than programming, The Structure of Magic Volume 1: A Book About Language and Therapy by Richard Bandler and John Grinder. It breaks down the actual skill of eliciting communications of fact that (competent) clinical psychologists use.
It's also great for putting PHBs in their place, as in
"What specifically do you mean, sir, by object oriented", and watching them proceed to flounder ... heh.