Don't just shoot for yes or no answers from your customers, give them the opportunity to really share their feelings from their interaction with you. Areas where improvement is needed, or where praise should be given, will be far easier to spot, thus allowing you to really make valuable changes.
Giving your clients the opportunity to voice their opinions after conducting business with you is a great way to express your interest in continuing to work with them. Just make sure you're earnest in hearing their thoughts and that you don't simply think this is accomplished with a survey alone.