If you're planning to travel by car with your pet, you should prepare in advance. If your pet has never been in the car before, make some short trips first to see how it reacts. Some animals behave just fine, but others suffer from anxiety or even motion sickness. If the latter is the case for your pet, talk to your veterinarian about appropriate medications. While you're at it, make sure your pet is up to date on vaccinations as well.
Most cats feel safer in the car if they travel in a small crate. To prepare your cat for crate time, leave the crate around the house for a few days so the cat gets used to it. Same with your dog. If you plan on harnessing your dog or keeping it in a crate in the car, get it familiarized with that feeling.
Make sure your pet is wearing its identification tags in case it manages to escape you during the trip.