Producers need to be involved in determining who should be invited, helping to identify the top prospects and clients. Everyone should be involved in following up on invitations. One month after the invitations are mailed, you need to call people to see if they're planning to attend. Then make a second call two days before the event. And, as was mentioned above, there's follow-up after the event as well.
Seminars can be a tremendous marketing tool if they are executed with care.
But this is not something that should be entered into lightly. It is a commitment. However it can be a lot of fun, and it is a great way to get the entire agency staff working together toward a single goal. Ask the audience to complete at conclusion of the seminar. Tell them you will collect them and pull one or two for a door prize.