I do not offer an incentive and just hope some complete it. If a person takes the time to complete the survey than they can expect their service to only get better. If your the type of person who would blow it off and then complain that is something is not to your standards than well...It is your fualt. Those seeking to stay with and want to help you improve will complete the survey. If you offer an incentive in my mind just makes it seem as if your you need their opinion more than anything and also makes it seem that you will offer an incentive for anything you need from your clients.

Sure, I might offer an incentive depending on what I am asking of my clients, but when it comes to a survey I leave it up to the clients to do the right thing and help out.