Just read this post by Philippa Gamse if you want to know what does and, importantly, what does not work when it comes to online satisfaction surveys (a must-read if you do your post-con surveys via the Web).
What does work is dangling a carrot to give people a reason to keep on slogging, in Philippa's case, free miles for completing an airline frequent flier program survey. She gives four examples of things that do not work as well, which to me range from 8 to 10+ on a 1 -10 scale of annoyingness. Can I add one? I hate it when, after actually going through the whole thing, I continue to get e-mails reminding me to complete the survey for weeks afterward. C'mon, have a mechanism to take those who did the right thing off the reminder list.