When providing activities and toys, be sure they are age appropriate and if you have more than one child that all toys are the same or similar to help prevent arguments.
Having snacks is a must, nothing worse than hungry children and not having anywhere to stop. Try to steer clear of high sugar foods, although having treats on hand to reward good and patient behaviour is always a winner for us.
Be sure to take toilet breaks when there is a toilet available, if you're driving pack some tissues in case of emergency toilet breaks etc (for girls especially), also allow the children to drink enough fluids, but not excessively or this could mean constant toilet breaks which is also a time waster!
For younger children they enjoy games such as eye-spy or spot the car (pick a colour each) and the first one to see a car that colour wins. If you're taking electrical distractions such as iphones, ipods or gaming consoles be sure to pack a car charger on long trips too.