This is a great question. One of the best persuasive posters that I have ever seen was Apple computer's "think differently campaign." There are three reasons why I thought the posters were great.
First, they were extremely simple. Please see link below. They were just pictures of famous people who have done something remarkable in life. Just the faces of the people show something great. This is really a case where a picture is worth a thousand words.
Second, by using these famous people in their advertisement campaign, they have made a mental association with these famous people and their company. This can be powerful, because the implication is that these people would have used an Apple computer, if they had the chance.
Third, it also leaves the impression that if you really want to think outside the box, then you must not use a competitor's computer (a PC).