Use any industry or high-profile connections you may have to establish and promote an advisory board. Use "big name" speakers or co-chairs along with their photos on all of your promotional materials. Familiar names, recognizable faces, and official-sounding entities will go a long way to helping your attendees associate your event with greater credibility.
You’re in Good Company.
Find yourself a reputable sponsor or co-sponsor and then shamelessly promote your partnership. If you have not yet established a reputation, "borrowing" someone else's can give you a tremendous attendance boost. You're smiling to yourself, but it's true! Think about what happens when Martha Stewart, or Warren Buffett endorses a product, service or company -- people recognize those names and associate them credibility (in certain industries, anyway).