That's the title TRIAL Magazine gave to my article in their July 2008 issue. That link is to a .pdf copy uploaded here, so you don't need to be an AAJ member to read it. (In fact that's the only way I can read it myself; I do too much defense work to be allowed in AAJ. TRIAL's generosity in letting me post it is just one reason why it's a terrific magazine to write for, and to read if you can get a copy.)
The article actually begins with what a mock trial can't tell you, before turning to why they're invaluable anyway, and a few tips on how to plan a simple, useful mock trial. I'm a believer, and in the article I try to explain why.