You won't waste time trying to fit that 10-pound bag of charcoal briquettes into the trunk, and you'll have more flexibility about where everyone sits. To maximize your seating options, take your own car seats with you, even if you get a van with those nifty built-ins. Little heinies can get sore when they're in one seat for too long, and it's great to be able to pull over at a rest stop and let the children switch. Get enough sleep the night before. While it's good for the children to be well rested before the trip, it's imperative that you be.
Sleep deprivation makes for frazzled nerves and unsafe driving.
Plan your travel time around your child's sleep schedule. This works if you're not too tired to drive late at night, and if you're reasonably sure your children will stay asleep once you arrive. Relax about snacks. And second, let the usual rules about junk food slide. Don't forget to pack snacks for the adults, too.