GREAT question, Bruce!
Word-of-mouth advertising is considered the most effective form. It has the desired qualities of strong credibility, high audience attention levels, and friendly audience reception. It features open-ended conversation with questions and answers about the product, psychological incentives to purchase, memorability, efficiency and frequency. Word-of-mouth advertising passes product information to many other potential buyers (and may even include promotional trial demonstrations and free sampling), at little or no cost to the business. Whenever possible, a small business should build an advertising program that results in word-of-mouth advertising. Satisfied customers are your best advertisements.