Raffy has just recently had his first few rides without getting sick at almost 8 months old. We have mainly kept his car rides short with a few exceptions because of the car sickness. Now that he's doing better, we're extending the car rides. He went on a longer one today, with several stops and did not get sick. At 6 months old, he was still getting carsick half the time.
My Rocky is 12 weeks old and has been on several car trips - the longest one so far being 2 hours one morning and then 2 hours back that night. He has done fine so far but I do try to be sure he's had lots of fresh air before he goes on long rides as this tends to tire him out so that he sleeps more and I also try to slow down around curves and so forth. In other words, I try to be a more aware of the motion of the vehicle. I'm not sure that this helps, but it makes me think I'm doing all I can.
Some puppies never get car sick, some outgrow it, and unfortunately, a few never do. For those, Dramamine (if drowsiness isn't a problem) or Bonine can help. I have a friend with a show bitch, who had to give her Bonine every time she got in the car. It's not sedating, so she was still up and ready to show when she arrived.
For SOME dogs, there is a psychological component. Especially if the owners avoid taking the dog places because of the motion sickness, and set up a cycle where the only time the dog is put in the car is to go somewhere they don't like. (Like the groomer or vet) this is easy to prevent, with lots of short rides to fun places, like class to a play date or for a walk in the woods.