The Black Swan . . . . .
The Impact of the Highly Improbable
by Nassim Nicholas Taleb
Humans will believe anything you say provided you do not exhibit the smallest shadow of diffidence; like animals, they can detect the smallest crack in your confidence before you express it.
The trick is to be as smooth as possible in personal manners. It is much easier to signal self-confidence if you are exceedingly polite and friendly; you can control people without having to offend their sensitivity.
The problem with business people is that if you act like a loser they will treat you as a loser--you set the yardstick yourself. There is no absolute measure of good or bad. It is not what you are telling people, it is how you are saying it.