When we get emails/letters that we could use as a testimonial, we just send a quick email to thank them and ask if they would mind us using it. If people have had a good (or even bad) experience with a company, they tend to tell otehr anyway, but it is courteous to ask!
Another option is to send a customer satisfaction survey out (we do this on the completion of all projects) asking for feedback and how the service can be improved/whether they had probs.
We have a really simple one, that asks 3 or 4 questions about the service with 'excellent', 'good', 'fair', 'poor' 'very poor' type answers and then space for additional comments below.
On the accompanying letter or email, we ask them to spend a few minutes completing the questionnaire and make them aware that we may use thier comments for testimonials.
