My vet recommended Dramamine for our dog who gets carsick but for now she is too small (only four months old and 3.5 lbs) for us to get the dosage right so the vet gave us a liquid medicine instead. I'll have to look at the bottle...can't recall the name. We've only used it once for a 2.5 hour car trip, but it seemed to work. The hard thing with Karli is that she doesn't seem to mind the car rides themselves or have a negative association with them which means that most of the above listed recommendations didn't work for us. She's perfectly happy in the car until she gets sick, then she throws up and goes right back to being perfectly happy until the next time she throws up. Not feeding her before a trip only helps in that she only throws up bile (and in smaller amounts) rather than throwing up massive amounts of dog food!
Good luck! Having a car sick dog is the worst; we visit my family every month and they live 2.5 hours away from us...it sucks!
-C