Short Cut your questionnaire to the bone. And then make it even shorter. The greatest mistake made by most Web surveys is to squeeze every possible question anybody could think of into a single form. Keep the survey to a single screenfull. Don't work either.
People stop after a few screens because they get tired.
Web users have very little motivation to help you in your project. They are on the Web for their own purposes and prefer to move right along and ignore your survey if it seems as if it will intrude on their time. Getting a high response rate is crucial for the validity of the survey. If only a few, unrepresentative users answer, then you might as well toss a coin. Incentives will improve the response rate, but you can't buy responses.