Idea: What if you offered an incentive for clients to fill out a survey/feedback form and bring it back for a discount? It could have questions like, where did hear about me? What about the ad/listing made you decide to call? Several yes/no closed questions, such as, was the office easy to find? Did you feel like your needs and expectations were addressed? Would you recommend my services to a friend with a similar issue?
For me, I'd add a few about my online features: Have you visited my website, and if so was it easily navigable and clear? Did you know you can request appointments online? (Yes I did that for my appointment/Yes but I haven't done that yet or don't have internet access/No, I didn't know) Did you know that gift certificates are available through my website?
Then at the end, optional space for other feedback AND an OPTIONAL testimonial. The incentive is for just completing the above portion, not necessarily the testimonial.
Yeah?