First is entertainment. Music, movies, and books for the whole family. You've got to have music to keep you grooving all the way there. Make sure your selection is varied enough to last the whole way and to satisfy your every mood. Bring some sing along music if you're traveling with kids, make sure it's something you can stand to listen to again and again and again.
Along that same line, you've got to have the portable DVD player if kids are in the car. (Head phones for each kid are not essential, but really, really nice!) And don't forget books on tape, or CD, which is what I recommend. They are a great way to entertain the whole family and perhaps interject a moment of classic literature.
Be Prepared
Next, remember the emergency supplies. A map is a must have as you may come across road construction, a natural disaster (mud slide, forest fire, etc) that will send you out of your way. You can waste hours if you miss that turn after the detour! Also, a good emergency travel kit with food, water, flashlight, and a well stocked first aid kit for anything unexpected.
Don't forget to have your engine fluids checked and make sure you have wiper fluid, a spare tire, and anything special your car might need. A cell phone is a no-brainer, and antibacterial wipes are always handy.
Food
Snacks are a natural part of a road trip. My family is partial to Red Vines and Peanut M and Ms. We always carry water and make a point to refill the snack bin along the way. If you're traveling during extreme weather (hot or cold) or into remote areas, carry some extra food and water, there have been enough stories in the news about stranded travelers, that you'd be silly not to.
Memories
A road trip forces you to spend time with the other people in the car. Make that time special and happy by being prepared and making the effort to connect to each other (that means turn the electronics off from time to time). Don't forget your camera. So often in our hurry to get there, we forget the adventure along the way. Stop at the view points, take a side trip to ah historic landmark, have fun along the way and record it in pictures.
