If at sea, lie down and close your eyes. If in a car seat, rest your head back on the headrest, so it is somewhat faced upward. This relaxes the semi-circular canals. Charcoal tablets may help settle the stomach. Ginger capsules are highly recommended.
Prevention is the best.
Do not eat heavily processed meals, drink liquor, or eat junk food. Some people do well eating a few whole-grain crackers before and during the trip. Avoid smoke and food odors. You may wish to take 5 charcoal tabs, one hour before the trip begins. Another method is 2 ginger tablets every 3 hours, beginning one hour before the trip.