Allow the kids to kick off their shoes. Bring pillows and blankies. Don't forget sunscreen and sunglasses. Take along enough for everyone in the car. Consider sun shades for the rear side windows.
Music soothes the soul.
Take turns driving. Switching roles can break up the monotony for children and adults alike. If your older kids are fighting or are just sick off each other, take turns allowing each one to sit up front. This is not safe if your children are younger than 13. If you have a baby or a young child, they also appreciate some mommy or daddy company every now and then.