It may not be possible to prevent it altogether, but you can make it less severe.

Before the journey, avoid rich or heavy foods and alcohol. Car sickness is a form of motion sickness, so it will be worse if you are in a vehicle that is jolting over a rough road or has an uncomfortable seat. So try to get a seat in the front, and make sure your seat belt is very secure to minimize movement. Never read in the car - don't even read a map if you can help it - and keep looking at a fixed point in the distance rather than changing your focus from inside to outside the car. If it's possible, get out of the car for a short break and some fresh air at some point during a long journey; if not, try opening the window for a while.

If you have severe problems you could ask a pharmacist to recommend some tablets or a remedy such as a wristband. These don't work for everyone though, so you may have to keep trying different things.