I've never seen this advertising tactic used by trade shows, but it's one used by several consumer shows in our neck of the woods: Posters in windows of local businesses.
For some strange reason, local businesses don't often care about how their window real estate is used. In Mt. Lebanon, PA, where our coffeehouse is, you'll find posters for for-profit theatre performances, circuses, and other downtown attractions, in addition to the expected charity and fundraising events.
Needless to say, nobody from Mt. Lebanon goes downtown to the theatres to advertise events taking place in our suburb. Nor do the store owners who offer space for these posters get a discount on tickets or a mention in the programs. Nope, they do it for nothing.
That's correct -- your for-profit events can be advertised by local businesses near your venue at no cost to you.
Just go into the store with your poster and ask to hang it up in the window. More than half the time the manager will say OK, often without even considering the content.
I can't explain why this is. As a part-time retailer myself, I don't give up window real estate for free for no reason. It had better be a cause we believe in, or something that's important to a good customer or one of our baristi. But to give away free ads without asking for anything in return? That's pretty dumb.
But hey, as long as people are dumb enough to let you advertise for free in their stores, why not?