The best way to do this is to have a channel for it that's very easy for people to use. If you all share an office, set up a time. If you all have access to the internet, set up a web form. If you're all in the same physical space for a short amount of time, give out little cards and have people write ideas and put them in a box.
Then if you really hate the possibility of improving you can throw it all out. But really, why pass up the opportunity to improve? Feedback is a gift. People take the time to make it better.
Do not be defensive. We have a human tendency to want everyone to think we're perfect, and if anything is not perfect it's not our fault. If your first answer is "no", take a minute to think about your second answer.
Do not tell them "I worked three months on this banquet if you don't like it then YOU join the board of directors and go to every stinking planning meeting." While dedicated volunteers are nice, do you really want people to feel like they have to attend a weekly meeting for three months just to tell you "there should have been more bathrooms" or "I couldn't get to the hot dog line without getting out of my wheelchair"? Most people won't do that, so if that's the barrier you are setting up, you just don't get to hear their ideas. You miss out.
And the more blocking or defensive you are, the more you send the message that the things you do Are What They Are, no changing or improvement.
Sometimes people will put huge weird things in the suggestion box, like "we should have also had a hot air balloon lift" and that's why you can ask for contact info and if they want to volunteer to help out, tell them you think it's a great idea, they have your full support in organizing the balloons! But at least hear their advice, and try to find a path to Yes.