I have had some success with ginger root extract (tablets or capsules -- you can get them somewhere that sells natural supplements). I give mine each a tablet or capsule about an hour before we take a long ride and it seems to help a lot. Any closer to riding time, and it isn't nearly as effective. Belle would actually eat the tablets right up; she liked them. I haven't been able to find them anymore and have switched to the capsules, which I usually have to hide in peanut butter to get them to take.

Also, now that I'm taking Belle for agility classes, she gets excited to ride rather than scared of it or sick, since she knows that she will get to go and play (and play hard ). This has helped a lot. If the pups think that the car means going to the vet or groomer or something else out of the ordinary/stressful, of course they won't want to ride.