Avoid long trips alone.
If you must make a long trip, plan a rest period every 160 km (100 miles). Get out of the car and walk around for a few minutes. Take time to use de washrooms.
When possible, use the main highways. It will be easier to find areas to stop if you need to use a washroom or a telephone.
Bring a small cushion to support your back. The trip will be less tiring.
Think about using the train or the plane instead of the car. These two means of transportation may be less tiring than travelling by car.