This should be given to your dog by mouth one hour prior to departure, and the dose depends on your pet's weight. Ask your veterinarian how much to give. Your vet can supply stronger medications, such as acepromazine which has the additional effect of sedation. I would only recommend this for long trips and it should not be given two days in a row. For natural remedies, try peppermint tea or bach flower remedy.
Peppermint tea is available in capsules and the bach flower remedy is given via dropper.
Both are given orally every two hours to combat excitement and subsequent nausea, and both are available in health food stores. It also may be helpful to encourage your pet to lay down on the floor of the vehicle or to keep them in a well-ventilated travel crate if they are small enough. It is best to keep your pet on an empty stomach while traveling, so withhold food about six hours prior to your trip. Water, however, should always be available before and during your trip. On long trips it's a good idea to pack a water bottle and bowl.