Word of mouth is very powerful in getting you business and a testimonial can add to that considerably.
Potential customers like to know that you can be trusted to give them a good service and genuine testimonials in your literature and on your website are a powerful tool. On your website you can have a feedback form where customers can easily leave a testimonial that can be read by others. Make sure that you have the permission of the customer to display it. The same is true if someone writes to you a letter or email. Ask their permission to display the relevant content as a testimonial.
Personal details like their full address should usually be omitted. They may even request that just their initials are displayed rather than name is used but a name is better from your perspective. People are less likely to trust a testimonial from Mr S of London than Mr John Smith of London.
Testimonials in Sales Letters
When you use testimonials in sales letters wherever possible try and use them from people that live in that area. This will add more value in the potential customers minds. If possible a photograph of the people is even better.
Never make up testimonials. If you are doing a good job there are ways to encourage people to write to you or leave genuine testimonials on your website.