This "We try harder" ad for Avis was developed by Doyle, Dane & Bernbach and is considered part of one of the  top 10 advertising campaigns by Advertising Age.

"The truth isn’t the truth until people believe you, and they can’t believe you if they don’t know what you’re saying, and they can’t know what you’re saying if they don’t listen to you, and they won’t listen to you if you’re not interesting, and you won’t be interesting unless you say things imaginatively, originally, freshly." 

 – William Bernbach, advertising man

Quote is found in "Hey, Whipple, Squeeze This" by Luke Sullivan>>