Make sure your dog is accustomed to car travel. Your holiday excursion shouldn't be his first trip in a car.
If you know he's likely to be sick or excitable in the car, you can get him used to the car journeys gradually.
This can be done by letting the dog just sit in the car without the car moving. When the dog is comfortable with this, take the dog round the block, and gradually move up to taking the dog on journeys for 5 minutes, 10 minutes etc.
Even though he may love to do it, don't let your dog hang his head out the window of a moving vehicle, as this can lead to eye injuries and may cause an accident.
When the dog is in the car, there are a number of methods of restraining him that you can use.
You may want to consider using a canine crate or have a separate area at the back of the car for the dog.
Alternatively, you can buy a dog harness, which clips into the seatbelt in the car.
Make regular stops to allow your dog to relieve himself if necessary and offer him water to drink.
Keep him on a leash when you exercise him, and never let him run loose at a service station or near a busy road.
Don't leave your pet in a parked car in hot weather.
Make sure he has some drinking water. Leave the windows open wide enough to allow fresh air in but not him out! You can actually buy window attachments that allow the window to be left open, without your dog escaping.
Even in dull weather, don't leave your dog in the car alone for more than 5-10 minutes.