Don't forget to pack a leash! Depending on the distance you and your pet are traveling, you may have planned out a few stops along the way. You'll need a leash to keep your pet from running away -- especially common in new surroundings -- during those rest stops. That leash will also be needed during stops you couldn't have planned, such as accidents or breakdowns, to help keep your pet close by and out of traffic.
Regardless of how far from home you travel with your pet, all pets should always have current ID on them, either tags or a microchip, with up-to-date information including whom to contact in case of emergency.
Not all traveling pets go missing because they run. Some people want your pet as much as others want your sound system: Pets left alone in cars are at risk for being stolen. Keep your pet on a leash and with you every time you're out of the car.