Sudden jerky movements tend to be worse for provoking motion sickness than slower smooth ones, because they disrupt the fluid balance more. A 'corkscrewing' boat will upset more people than one that is gliding smoothly across the oncoming waves, and cars driving rapidly around winding roads or up and down a series of hills. Looking down into your lap to consult a map or attempting to read a book while a passenger in a car is another common cause of motion sickness.
Many 'cures' and preventatives for motion sickness have been proposed at various times. One which is both practical and effective is to simply look out of the window of the moving vehicle and to gaze into the distance towards the horizon in the direction in which you are moving. This helps to re-orient your inner sense of balance by reaffirming to your inner ear that yes you actually ARE moving. Fresh air blowing on your face can also be a relief.
Other cures for motion sickness rely on medication. Over-the-counter and prescription medications are readily available, eg. dramamine. Ginger is a mild anti-emetic and sucking on crystalised ginger or sipping ginger tea can help to relieve the nausea.
Astronauts suffer a form of motion sickness called space sickness (or space adaptation sickness), caused by the lack of gravity disturbing their sense of balance and the endolymph fluid in their inner ear.