I spent the bulk of the past few days in meetings...and while they were interesting, I think you lose people after 45 minutes.
Perhaps it harkens back to how long your high school classes were or something, but a 15 minute break every hour would, IMHO, make a big difference in getting valuable participation from your attendees.
Obviously, attendees should come back on time (more on that later) and there are times when there is just a lot to cover, but more often than not, if you are looking for fresh ideas, give people a chance to regroup.
If I were the godfather of meetings, I'd integrate these suggestions on how to conduct meetings