Dear …
We are privileged to have been able to design a __(marketing campaign/brochure/website)__ for you and are pleased that you are finding it to be producing the kind of results you were looking for. As you know, our team of professionals go to great lengths to ensure that we understand your industry well in order to create and incorporate the kind of concepts that work well for your specific business. That is why we make it our business to know yours.
It is with pride that we work hard to achieve the kind of results that our customers are happy with and we are delighted to know that you are pleased with our__(product/service)__. In keeping with this, we are asking you to please take a moment to share with us, your testimony endorsing the kind the __(product/service)__ we offer. We would greatly appreciate it if you would be willing provide us with a written testimony, attesting to the positive ways you have found that our __(product/service)__ has worked for you.
Thank you for taking the time to do this for us and once again, we want to extend our thanks for having allowed us the opportunity to show you how our expertise can work for you.
Yours sincerely,