Oh, car sickness, I know it well. Our Otis will puke all the way from Michigan to Pennsylvania if we don't medicate him. He's okay on short trips (less than 45 mins), but longer trips are pure torture for him and us.
If the dog is young, it may be something that s/he grows out of. Or they may be able to do some serious conditioning to train it out of the dog - sit in the car in the driveway, turn the car on in the driveway, pull out of the driveway then right back in, go for drive around the block, etc etc etc. I didn't have the patience or time to do that with Otis, so we did the meds route.
1) Dramamine - didn't work
2) Benadryl - didn't work
3) Phenergen (promethazine) - didn't work
Finally, we hit on a really new drug specifically made for nausea in dogs - Cerenia (maropitant citrate). It works GREAT. Before a long trip Otis gets nothing by mouth for 3hrs before the trip, then he gets the medicine 1hr before we hit the road. It's somewhat pricey at $5/pill, but so totally worth it.