It seemed to me that every conference I attended had good attendance. Have a wide range of topics present. Provide plenty of networking opportunities. Be as flexible and acconmodating as possible. Reduce the costs.
Lodging is too expensive.
Provide incentives for presenters. Possibly offer reduced rates for presenters. I don't know how much marketing is done to non-member institutions. The accomodations on campus are often limited and we may be better served if we went to a conference center. Get a good mailing list and use it for advance publicity.