I think that considering the virulence of norovirus, you could probably expect 150-300 people to have caught it without attributing it to bad practices. It's just very hard to prevent the spread of something that survives weeks on a dry surface, is resistant to everything except bleach, and only requires that you ingest 12 virions before you're infected.

The biggest thing you can do is just educate people about handwashing - I think everybody knows that they SHOULD, but many assume that hand sanitizer works, and don't have any idea of what's involved in a proper scrub. Maybe put signs like this one - http://www.health.state.mn.us/handhygiene/wash/fsgermbuster.pdf -up on the bathroom mirrors?

It is probably good that this happened so that next year the staff and volunteers are aware of potential issues due to the near miss.