AskPatty's Tips for Driving with Dogs -- Tip #7
DO: Be sure you have Pet ID!
Make sure your pet has a microchip for identification and wears a collar with a tag imprinted with your home address, as well as a temporary travel tag with your cell phone, destination phone number, and any other relevant contact information. Canines should wear flat (never choke!) collars, please.
If you're traveling across state lines, the ASPCA suggests you bring along your pet's rabies vaccination record, as some states requires this proof at certain interstate crossings. While this generally isn't a problem, it's always smart to be on the safe side.
Be sure you bring a photo of your dog with you or have some on your cellphone, in case you need to identify your dog as your own.