If you ask them where else they shop, list all the competition. If you ask them what newspapers or radio stations they listen to, list all of the popular ones with a space for other selections. Offering choices makes it easier and less uncomfortable for the customer trying to be really honest without hurting your feelings. In actuality you'll get more honest and useful answers by providing the choices than by letting them fill in the blanks. Decide what you want to find out.
Any good survey has a purpose in mind.
Make a list of the top 5-10 things you want to know about your customers. Review the list and decide if, and how, you will benefit from the information you gain. The only real way to control the amount of responses you get back is to have someone ask your customers the questions and then record the answers. Personally, I feel that hiring a telemarketing company puts you in the same league as those irritating, always catching you at dinner time, this is what caller id was intended for, pests. At one of the garden centers I consulted with we hired a high school student to give the interview.