You just have to know and believe in your product.
That works great if you have an excellent product with a proven track record. It's like being a BMW car salesman versus a Yugo car salesman. BMWs pretty much sell themselves because of the quality, workmanship and sporty appeal. Yugos, well, you pretty much had to bullshit your way through that sell.
edited to add: of course if you believed that Yugo was a better car, I guess it wasn't technically lying, just not properly informed and out of touch with reality...kind of like JWs trying to sell the WT religion...