Ah, now I'm not sure... Is the the idea to encourage people to fill out the questionnaire, and perhaps thank them for doing so? Or is it to solicit future communication from people? I thought it was the former.
Also, on second thought, but still in keeping with my original understanding, how about "Thank you for your feedback"?
It means we are there to hear you out if you need to get in touch with us because you need further information or have problems. When you answer the phone you can say "Je vous écoute". so the idea is definetly "Don't hesitate to contact us".
It's more, "We welcome your feedback" I think. Because what I want to do is encourage people to fill out and send the form but not contact us by other means.
I just want to say, "we will listen and consider what you say if you send it to us" ...
Actually, rereading you post, I suppose we are talking about a service rather than a product? If it is ex. Yahoo which has answered a question about problems with your blog, I would say: Don't hesitate to contact us if you have any further questions."
If it is about a product you might add"...or should a problem arise."
Ok. (As one of my students once said, "It's so easy when you understand!." ) Then just go for "Merci de votre avis qui nous aidera à mieux vous servir à l'avenir." "Thank you for your feedback which will help us to better serve you in the future."