It's such a simple concept, you might not think of it -- telling your kids ahead of time how you expect them to behave. It's good to give children boundaries, so share each day's agenda with them and explain the family rules. For example, you might say you'll be driving from 9 a.m. to 4 p.m., stopping for a picnic lunch around noon. The kids will each get to bring two of their favorite toys, and you'll supply snacks and water. Each person can choose music for an hour and can't complain about someone else's choice. Everyone must use the restroom during stops, and you'll take away the toys of any child who misbehaves.
What rules you set will depend on your children's ages and temperaments, of course, but it's helpful to everyone if expectations are clearly laid out.