Our advice remains: never pay for an ad to promote your show. A paid ad rarely produces enough return to justify the cost. Yes, use barter ads to drive traffic to your web site. And we recommend you not run a 16-page show preview in a magazine. This only wastes trees. Better to do a nice 2-page spread that pushes prospects to your web site.
There is a new book titled The Fall of Advertising and the Rise of PR (by Al Ries and Laura Ries). We agree that good PR is much more valuable in promoting shows than ads. Some thoughts on good PR:
You are trying to build the buzz for your show. Thus, the object is to get positive mentions pre show from those who influence your target audience (usually this totals no more than 20 individuals).
These 20 influencers include editors and beat reporters on B2B magazines, association/society staff and industry gurus (e.g. consultants, professors, etc.).
Do this yourself. Only you know your show and how to develop a rapport with these 20 influencers.
If you do decide to go outside, use someone who knows your industry, rather than a generic PR agency. You will spend too much time teaching them what widgets are.
Your show itself is not newsworthy. Instead, send releases that say, for example, "Widget Expo Keynote Speaker Jane Doe predicts a 7.3% rise in widget production during 2003."