This was posted in a friend's journal, and she reposted from two other people's LJs. I thought this is something appropriate to this community, since we attend cons and events, and for safety's sake, these are good ideas.
Suggestions for those attending meetings and conventions from fabricdragon and thorninthefoot
Write on the back of your badge
an emergency contact number
any serious allergies
any serious medical conditions
needed info (ex. "my epi pen is in my purse" or "my heart pills are in the skull charm on my wrist")
If you have serious health problems, the suggetion was to print up the information on a sheet of Avery Labels to take with you, so you can just slap it on the back of your badge. The suggestion was made that blank labels with headers (allergies, emergency contact) could be made available at conventions -- and that convention/meeting attendees should have this suggested to them on check in when they pick up their badges. And you could write down "my cell phone number" so you could get your lost badge back. I write my email address on/in my program so that if I loose it, it will come home. I tend to take copious notes in mine!