I just found this quote attributed to David Ogilvy, in his book Confessions of an Advertising Man:
People do not buy from bad-mannered liars.
Well, not twice, anyhow.
Considering that David Ogilvy is sometimes called the Father of Advertising, one might do worse than think about his statement.
For some reason, I feel like all of my thousands of words on Cluetrain marketing and so forth just got summed up in those eight words.