Clancy Martin used to be in the jewelry business, and he'd often lie about the quality of the diamond he was selling. His trick:
If you want to be an expert deceiver, master the art of self-deception. People will believe you when they see that you yourself are deeply convinced. It sounds difficult to do, but in fact it's easy -- we are already experts at lying to ourselves. We believe just what we want to believe. And the customer will help in this process, because she or he wants the diamond -- where else can I get such a good deal on such a high-quality stone? -- to be of a certain size and quality. At the same time, he or she does not want to pay the price that the actual diamond, were it what you claimed it to be, would cost. The transaction is a collaboration of lies and self-deceptions.
(Hat tip: Kottke)