I actually specialize in promotional products, and I was in a meeting yesterday with a major player in the industry. He mentioned something that I hadn't thought of, but I'm sure it's been done.
Rather than waiting til the people are actually at the expo or at the tradeshow, reach out to them before hand. If you have a list of people who are going to be at the tradeshow or people you want to get there, send them something beforehand. It could be a coupon for a free t-shirt, or an entry to a raffle. You're building a relationship and peaking their interest. He suggested sending them a promotional jotter with the first page imprinted as a coupon for a free tote bag or t-shirt.
I thought it was definitely interesting.