Best Answer - Chosen by Voters
Your body while in a moving vehicle, like many peoples, is confused. When you look out the window you see that you are moving but your body is still; your ears sense motion but when they don't and your eyes see that you are moving your body gets very confused. The more motion there is in the vehicle the less likely you are to get motion sickness. On a boat its the opposite, you are moving extremely slow (according to what you see) but the boat rocks back and forth indicating movement; this causes the same problem.
To get over this problem looking out at things that are not moving helps when in a car, like looking out at the horizon; this way both your ears and eyes can agree. While in a plane looking at the ground below can cause motion sickness so I would advise to keep looking up or just keeping the window closed. Ginger can also help a lot with nausea, ginger candy is a must have for long trips like that even just in case someone else needs it (you don't want to get puked on!). Another thing that you could do is get a motion sickness bracelet. Its a bracelet that puts pressure on a pressure point on your wrist that keeps you from getting nauseous. You can wear it all the time and I think its easier than carrying around ginger candy or focusing on not looking at stuff around you while in a vehicle.