While some people are much more susceptible to motion sickness than others, anyone can suffer from it, and most have experienced at least mild symptoms in their lives. However, by taking precautions, the incidence and severity of motion sickness can be reduced.
The misery of motion sickness is basically caused by the body being confused. To determine where it is, the body is constantly checking visual, touch and inner ear information (which registers balance) as well as expectations. When these information sources give contradictory information, the body experiences the symptoms of sea sickness.
For example, as the inner ear is being jostled when riding over rough waves, it signals the body it is off balance when it really is not. Or, when the eyes are reading a book in a car, the eyes are telling the body it is still while the bumps and other motions of the car are giving the inner ear contradictory signals. From this disagreement, which is human body is not evolved to deal with well, nausea, dizziness and other symptoms of motion system emerge.
As a general rule: the greater the motion the worse the sickness so anything one can do to reduce the motion will help. For example sitting in the middle of the plane or boat where the motion is less will help. First and foremost is to reconcile the contradictory feelings of motion and standing still by reducing the difference as much as possible.