The reality is that there is probably more chance a meteor will hit your show than a terrorist will attack you. However, the perception among many prospects is that large gatherings are susceptible, so you must address this concern.
To improve the perception of security at your show, be sure you take visible steps such as:
Uniformed guards in the registration area and at all entrances, including some armed.
Insure that everyone on the floor has your permission to be there (i.e., has your badge). At check in, require a photo ID of both attendees and exhibitors. Also, control your contractors and have the venue control their employees/contractors.
Some shows are not allowing attendees to bring their bags, briefcases, etc. on the show floor. If you think this is extreme, we suggest you do random searches instead.
Note there is an "unintended consequences" benefit of hightened security: the rampant theft problem at many shows will be reduced. To insure this happens, add guards to the loading dock and do random checks of containers going in and coming out. You will likely not find anything, which means you probably thwarted the bad guys (thieves as well as terrorists).
The Exposition Operations Society (EOS) has a 17-point checklist of security measures at shows. To obtain a copy, e-mail to Steve Schuldenfrei at firstname.lastname@example.org.