Motion sickness (whether it is in a car, boat, plane, etc.) is caused when there is a discrepancy between what your inner ears feel and what your eyes see. If the vehicle you are traveling in is moving around, but you look at a fixed point inside the vehicle, your ears will detect motion, but your eyes don't. This leads to motion sickness.
For children, this often occurs in the car when they don't watch the road because they're too short to see out or are doing something like reading a book or watching a movie. Many people get sick when they are a passenger in a car, but not when they are driving. That's because the driver is watching the road more closely than the passengers -- the driver's eyes and ears agree on the motion they feel, so they don't get sick. For divers on a boat, motion sickness often occurs when you're busy getting your gear ready because you're not watching the water, but the boat captain almost never gets sick because he's watching the water and the horizon the whole trip.
I was on a dive trip once where one of the women on the boat was getting seasick. She was convinced that when her symptoms started she should lean forward and place her head between her knees, staring at the floor. I tried to tell her that she should stand up look at the water and the horizon instead, but she didn't listen. Sure enough, she got even more sick and eventually "fed the fish."
When I was in graduate school I did some studies of motion sickness while working for the Army Research Institute. Tank drivers at Fort Knox, Kentucky, were getting motion sick during training in full-motion tank driving simulators. My research group was asked to assess the situation in an effort to prevent soldiers from puking in the sims, which took quite a while to clean. We tracked the exercises the soldiers were performing and had them do balance tests (like a sobriety test) and rate their motion sickness severity after using the sims. We found that there was a slight time delay between the computers that created the video images and the computers that ran the hydraulic motion system. This fraction of a second difference was causing the motion sickness. We also discovered, however, that the soldiers with more experience in the sims were less likely to get sick, which means that they were learning to deal with the motion delay. We also found that some training sessions (like driving in the mountains and loading the tanks in flat-bed trains) were more likely to cause motions sickness than others. We reorganized the training schedule to move those sessions to later in a tank driver's training course, thus giving the soldiers more time to get accustomed to the sims. This lead to a reduction in motion sickness.
So, it is possible to "learn" how to not be sick over time if you can gradually expose yourself to sickness-producing situations. When you start to get sick, be sure to look up and out toward the horizon. Stand next to the captain and watch the waves as they approach the boat. This will ensure that your eyes and inner ears are in agreement, thus reducing your symptoms.
