How to avoid getting motion sickness?

I got sick sometimes in mini-buses going very fast on windy mountain roads. It helped switching into the front seat next to the driver and watching the road ahead rather then looking out through the side window. I think I never got sick in the front seat.

Since anti-nausea medicine (anti-emetic) isn't too common on the road, if you are traveling and in the need of drugs because of motion sickness, a large dose of antihistamine (e.g. Benadryl) can help with symptoms.

If you are trying to avoid drugs all together, some remedies that have worked for me are:

1. Eat something. It's counter-intuitive when you feel like vomiting, but having a fuller stomach eases the nausea. Salty snacks that are light on oil seem to work particularly well.
2. Ginger ale, the more ginger content the better.
3. Carbonated drinks, preferably sugar-free. The bubbles can induce burping, which alleviates some of the nausea.
4. Sit where there is the least amount of motion in whatever vehicle you are on. In road vehicles, that is generally in the front; on planes, over the wings; on boats, near the bow; etc.

I used to have a major problem with motion sickness when I was a kid and I almost always ended throwing up when travelling by car (strangely, never on a plane or trains though). Often took medicines to combat motion sickness, eventually grew out of it. Mark's answer has summed up the causes which lead to motion sickness so I won't repeat them. Since you asked for a solution other than medicines, here's what helped me (your mileage may vary):

• I felt suffocated every time I sat inside a car; keeping a window cracked open helped.
• Not eating meals right before travelling, if I could.
• Mints (not gum or chewy mints though). When I used to get motion sickness, I would get a vile taste in my mouth which often led to vomitting. Mints helped, for some reason.

Another possible option: I have seen a few news articles recently about these motion sickness "glasses" - they are essentially some spirit levels on a glasses-like frame. The manufacturers claim you only need to wear them for 10 minutes until your brain gets used to the movement, though I have not tried them personally.

What helped me on a boat was laying down, get one ear on the bed or seat and relax. I could even fall asleep in that position. As soon as I got up from laying down I was sick again on that trip, felt sick on others where the motion sickness was not as bad.

In airplanes I have used one earplug, a trick I have heard about but can not confirm working, as I am not that often sick in planes.
Some people report that special bracelets or plasters which puts a item to the skin help. I have never tried those. Here is a link to an answer of someone who reports being helped by them.

One thing that did never help me, when it was still tried, was medication. Without I would be sick once every leg of a car travel, with medication I would be sick all the time in the car. But as I have not suffered being car sick (much) since the middle of my teens, I have not use medication as an adult.