The folks at Amiando did a cool study looking at the Twitter usage associated with event hashtags. There are a number of cool findings in the report, but the one that stood out to me was that the vast majority of the tweets took place during the event, with a long tail before and after the event.
What does this mean for event marketers? I think it means that you should make sure to have an event hashtag that is very easy to find at all times during the event. post it in the event rooms and on the event agenda and other materials. Make it easy for people tweeting to have the hashtag handy. The second tip would be to have some way for people not at the event to engage with your content - a lot of the tweets were from people not actually at the event, and a lot of the people reading the tweets will not be there either. So give them some part of the content to interact with online even if they are not there live. Putting presentations online or live streaming some of the event would be a couple ideas.