When you mail the final survey to the members, give a deadline. You don't want to be tabulating responses as they trickle in over a period of months. Analyze the response. A statistically significant response rate is a matter of opinion. Consider outside experts.
Surveys are part science, part art.
Also, members may respond more frankly to an objective, outside party. Act on the results. If survey results don't contribute to decision-making, the money and time are wasted. If the survey analysis validates suspected problems, accept it and take steps. If the results from a small sample seem totally off base, check the sampling methodology and do a retest.