Never ever leave your dog in a car when it's hot or really cold out. Remember that the temperature can rise very quickly in a car even with the windows vented and when it's really cold out, it's really cold in the car too.
Make sure you have plenty of water along for the trip.
Take a couple practice rides in the car with the dog to get him used to it.
Stop often until your dog gets comfortable traveling in the car.
Feed him a couple of hours ahead of time so the stomach has a chance to get settled.
Remember to bring plenty of his own food. You might not be able to get it at your destination.
Keep a spare leash and bowl in the car, just in case.
Lock your car (and take the keys) if you need to leave them in the car in case someone accidentally opens a door.
A favorite toy may occupy them and make them feel more comfortable.
Bring lots of waste pickup bags - help keep the community clean!