I have suffered from excrucitating motion sickness since I was very small, and a severe ear infection at around 12 years old only served to make matters worse. I have tried all kinds of things, some successfully, some not.

1. Sit in the front seat if possible and look
   out the windshield, not the side window. Try to keep your eyes on something unmoving far away like clouds, mountains, skylines or distant trees. Looking out the side where things are flying by keeps your mind trying to readjust to what it is seeing, and it will attempt to adjust the pressure and balance of the fluid in your ear, making you dizzy. If you are in the back seat try to sit in the middle so you can see out the front of the car.
   
2. Roll down your window or run the fan as high and cool as is comfortable. I'm not sure exactly why this works, but the movement of the air and the cooler temperatures help quite a bit. Fresh air that doesn't smell of new car, leather, stale or compressed makes a big difference too, but if you are driving through a congested city with lots of exhaust fumes, you may want to opt for the air conditioner instead.
3. Lay down in the back seat if things get bad. Close your eyes and put pressure on your forehead or temples. Most seat belts extend far enough that you can keep it buckled as well.
   
4. Do not lean your head back on the headrest. This may feel comfortable at frst, but after a little while, the vibrations of the vehicle will only make your motion sickness worse.
5. Eat Mentos. The miracle treat. There is something about the combination of mint and that particular chewy consistancy that works wonders. They even make them in sugar free for the figure conscious. Also works on boats, planes and carnival rides. If you don't have Mentos, a mint or minty chewing gum will help too, but Mentos I have found to be the most effective. If your child is quite small, bite off a half or quarter for them to chew on to guard against choking.

Things that haven't worked very well for me are:

1. Dramamine. Well, it does help, but it knocks me out for between 3-6 hours at a time. Don't use it if you don't want to be comatose the rest of the day. (But maybe I am just sensitive.)
2. Wrist bands. Don't do anything for me, but they are inexpensive, so give them a try, they may work for you.
3. Motion sickness pills. In the US these are by prescription only so a little hard to get, and when paired with a doctors visit, rather expensive. Mentos are cheaper.
4. A cool damp cloth at the back of the neck. Helps, but is hard to plan for in a car or airplane.
5. Ginger ale or sprite. The bubbles cause problems, since extreme motion sickness is often accompanied by gas and bloating.

If you know in advance there is going to be a problem, eat only lightly before you travel, and drink only a little and non-fizzy drinks. In a boat, stay outside on deck as much as possible, preferably where there is wind, and away from any fumes. For a long airplane ride, maybe you want the dramamine. There's not much to do anyway. The very best luck I have had is with Mentos. I always carry a pack in my purse, and a couple when we go to any amusement park. Plus your breath is minty-fresh afterwards!