Dogs generally become sick in the car because they are frightened, not because they have real motion sickness. It is necessary to reassure your dog that these fears are groundless (which might be hard depending on how you drive). You need to get your dog used to the car by taking trips that are short enough that your dog does not exhibit the typical signs of car sickness- drooling, vomiting, etc. It may be necessary to start out by just sitting in the car together and giving your dog a treat after a few minutes. Then take very short rides followed by a treat - even if you can only make it to the end of the driveway. Gradually increase the length of the trips until your dog enjoys the car rides.
If you don't want your dog in the car, but find it necessary on occasion, you can use tranquilizers to control the nervousness with good success in most dogs. Your vet will have one that works well for him or her.
Some dogs really have motion sickness. It is possible that dramamine will work in these dogs but even in these dogs tranquilizers like acepromazine that also have anti-emetic properties usually work well.