Take along plenty of familiar food. Don't forget to take along a leash. Make sure your pet has a collar with an identification tag having your name, complete address and your telephone number on it. Before taking your pet on a long trip, take them on some short trips, even just to the supermarket and back. This will get them familiar with riding in a car.
Pets get car sick too, so try to plan short trips with frequent stops.
Similarly, leave your pet alone in the car for short periods of time so it will learn you will always come back for them. If your pet is prone to car sickness, consult your veterinarian concerning what to give your pet, and carry a supply with you. When traveling with a pet it is especially wise to make reservations beforehand. Find out whether your pet can stay in your room with you. Many hotels and motels accept pets, and some even provide kennels.