Customer testimonials are some of the best endorsements you could possibly have. Ask permission to quote them in promotional material. If they say yes, then it is wise to have them sign a legal release (see Legal).
1234, Main Street
Boston, MA 02123
08 April, 2005
<Recipient Address Goes Here>
We couldn't ask for greater accolades than kind words from our clientele. <Source.>.
Would you give us permission to quote you in <where quote would appear>? Your name would appear <where it would appear>.
<Customer name>, you would really honor us by saying "yes." I'll call you <time of call> to get your decision. If you decide to go ahead I'll send you a release form. Thank you for your business and the kind words.
encl: <List of enclosed items goes here>