Testimonials are a common advertising method where customers attest to a product or service's effectiveness. For testimonials to be effective, companies should consider customers' views, keep promises, stay ahead of competition, offer great benefits, maintain loyalty, and build relationships. Testimonial techniques include celebrity endorsements, expert opinions, average users, and satisfied customers sharing their experiences. Testimonials are a cost-effective way to promote products and build trust as they provide real evidence from customers rather than just claims from the company.