These are really tough times to generate attendees who actually show up. Here are some techniques that are working:
Saturate your drive-in market to capture last-minute "impulse" attendees. This starts two weeks out and ends when the show opens. Bonus: late registrants verify much higher.
Be open to any qualified attendees. For example, definitely market to those who do not belong to your association. Bonus: these non members may join up.
At three weeks out, publish your actual pre-registration list (but only name, title, firm). This proves how good your show will be—to attendee prospects and to still-to-be-sold exhibitors.
And do not assume that all pre registrants will come. Promote to them right up till the end. For more on this, see Tip 10: Improve Your Verification Rate.