A variation of word-of-mouth advertising is what I call testimonial marketing. The best way to explain it is with some examples.
The best automobile salesman I've ever met is David Khan. This guy is a true professional in every sense of the word. He's knowledgeable, competent, a skilled listener. He's able to exert pressure without being offensive and has many other great sales skills but his most impressive attribute is mastery of testimonial marketing.