The first suggestion is manage the expectations of your exhibitors because they will cut you some slack during these tough times. And now may be the opportunity to fess up about your true (verified) attendance figures. This will free you from chasing numbers, so you can focus on quality.
Next, over communicate with your attendees and prospects. Send more direct mail, broadcast faxes and e-mails. Assure them that air travel is now safe, but offer them alternatives to flying. Be very, very accommodating by extending early-bird deadlines, allowing last-minute cancellations, etc. Customer service now trumps all else. Love your attendees!
Heavily discount conference registrations, such as making a 3-for-1 offer. Consider bribing them, such as the association (Direct Marketing Association) that is offering free air fares. Add relevant content at the last minute, such as "Post September 11 Outlook for Widget Demand." Consider appealing to their patriotism: "President Bush wants you to attend Widget Expo!"
Flash: Registration firm, Convention Data Services, says that last-minute registrations are verifying at 60-85% (vs. normal 30-60%) and verification is also higher for those who register online. Thus, promote right up to the show and keep your web site live even through the show. And do not penalize nor discriminate against these last-minute registrations.