Your pet should always wear their ID tag affixed to the collar. If your pet is not use to having a collar on, break them in a few days before leaving so they get used to wearing it.
Keep contact information up to date. If possible put your cell phone number on the ID. Also make sure to add your veterinarian's phone numbers.
Carry several recent photos of your pet. Make sure each photo is from a different angle so others can get a clear image of your pet.
Ensure the clasp on the ID tag is securely fastened to the collar. Some links loosen from wear and tear, check these often.
Register your pet with a pet recovery system such as Homeagain
If your pet has any medical needs be sure to include this on a separate medical alert tag.
Bring an extra collar and duplicate set of tags just in case one breaks or becomes lost.