People are more likely to do favors for those they know and like, so those trying to convince you of something try to appear likable, which often includes being physically attractive as well. In addition to the fact that people like looking at beautiful people, people also tend to attribute other positive and unrelated traits to the person as well. Liking also depends on similarity, since we like people who are just like us. Flattery really can get you everywhere. Association is important to, and is often used to connect the person to positive images or events.
Authority is yet another principle.
Many of you may have heard about the Milgram experiment in which subjects thought they were giving experimental confederates electric shocks in response to requests by the experimenter. The experiment showed that most people have a very strong tendency to obey authority figures. Moreover, in each instance, those individuals who deferred or obeyed underestimated the effect of authority pressures on their behaviors. Through this strategy the person creates a perception of honesty that makes all subsequent information seem even more credible to observers. We'd better watch out for this as well.