You could say something like,
We appreciate using our service and we hope that we performed to your satisfaction. We have a small questionnaire which will help us improve our services for out future customers. If you don't mind taking a moment to fill it out for us, we would welcome your opinion of our services and any suggestions on how we could improve them.
You can offer an incentive, like anyone who returns a questionair gets a free.....
I hope this helps. Let me know if you have any further questions.