What causes motion sickness?
Our brains perceive and process motion mostly by relying on signals received from our inner ears. Yes, it's all in our heads, but from a physical standpoint rather than mental. Vision and physical messages from below the neck are secondary in helping our brains determine motion.
Motion sickness most often occurs when our brains have to rely solely on the messages from our inner ear and don't have any cues from our eyes or bodies. It is also aggravated when we feel warm or hot, have a head cold other pressure-causing ailment (messes with the fluid in our inner ears), or have consumed lots of sugary drinks or sweets.
Here are the general rules for avoiding or at least minimizing motion sickness without using medications.
On the Plane
On the Road