Sometimes we spend too much of our effort promoting to the same old prospects, such as past attendees and members. Expand your universe by going to fresh sources such as trade magazine subscribers. But be sure to target those with a "predilection" to attend your show by using one or both of these criteria:
They have actually attended another show within the past 12 months.
They are in a new situation (changed jobs or got a promotion).
Also, use others to help uncover these fresh prospects:
Have another association or magazine actively promote for you (offer them a cash incentive for each verified attendee).
Use "word of mouth" by offering, for example, free registration for a first-time attendee colleague (two-for-one sale).
We, as an industry, have to bring more new blood to our shows. You should do your part with your own event.