On the RoadID site they suggest some things to put on there.
Here's what's on mine if you're interested as an example.

First and last name, year of birth
city, state and home phone with area code (we travel fairly often and something could happen when I'm out of town)
husband's name and his cell phone #
asthmatic, NKA, NKDA (no known allergies, no known drug allergies)
blood type, organ donor


My last line is personal: it says: Catholic- call a priest. If I'm dying I'd like my last rites at least. Morbid, but it's how I'd want my last moments to be taken care of.

If you have more medical history, you may want to include more pertinent info. You can have 6 lines of info.