What causes travel sickness?
For the vast majority of dogs, car sickness is related to stress rather than the motion of your car. Some say that your dog's most powerful memory is of the car journey that took it away from all it ever knew to be safe and secure, namely its litter mates and mother. So from a very young age, car journeys are often associated with bad things.
Like humans, dogs can also suffer from motion sickness. Motion sickness can be caused by a number of factors, but primarily it is by movement in the inner ear.
What can I do to prevent or cure car sickness?
First and foremost, it is important your dog associates car journeys with positive and good things. To do this, you need to acclimatise them slowly.
First, get your dog used to just being in the car, without it going anywhere. Everyday, leaved them in a well ventilated car for up to 30 minutes (building up slowly). Ensure they are comfortable and have a bed in which to sleep during this period.
Do this for a week and then start to do very short journey's (5-10 mins) with a really positive experience at the end of it - usually this would be a nice walk or a ball game in the park.
It's a good idea to have someone else in the car during these initial journeys. This person can sooth and distract your dog during the journey.
At the end of the return journey, create just as much fuss and play a short game.
Gradually build up the length of the journeys up to about 30 minutes. If your dog is sick during a journey, reduce the length of the journey such that it ends before they are sick. Build the journeys up again.
When your dog is able to do 30 minute journeys without stress, anxiety or sickness, you are pretty much there.