Ellie used to get carsick all the time and then it kind of let up. Well, it's back. Sometimes 15 minutes of riding will set her off and other times it takes almost 2 hours. A vet told us once that the lower an animal rides in a vehicle (best on floor) the less sick they get. I really don't know if that's true and am not making her stay on the floor.
I also have heard that giving them some Gingersnap cookie before leaving helps settle their stomach but I have never tried it.