A mobile internet hotspot: If you're one of the many attendees (like me) who will be writing, editing photos, and transcribing interviews during your time at the con, don't rely on the often-shoddy free WiFi set up around the con. Bring your own hotspot to save yourself the frustration of running back and forth between your hotel and the convention!
A safety pin for your badge: It's always a good idea to secure your entry badge to your clothes with a safety pin, just in case your lanyard gets knocked around or somebody tries to swipe it. Plus, you can ensure that it's faced the right way so that con staff won't ask you to flip it around every time you enter or exit the convention floor!
Deodorant: Con days are long and sometimes sweaty — do everyone (yourself included) a favor and freshen up in the bathroom every so often.