Ooohh! It is so horrific! You are making me dread this summer's trips already! (Okay, so I only have 2 - I know, I get no sympathy from you

- but I travel with my sister and her 4, in the same car, w/o husbands to Grandma's frequently. A 12 hr drive.)
I do okay with the ahead-of-time packing of luggage -I am a list person - It is the "in the car stuff" that drives me crazy. The stuff to keep them occupied (or at least form driving me crazy) during the trip, - the games, crayons, paper, snacks, books, dolls, AAAAAH!
One thing that really helps is that I have a list for each family member, myself included. Very detailed (right down to mascara, comb, underwear - things I know I'd never forget. But have forgotten.) And for several types of trips: camping trips, road trips, etc. What they need in the car, what they need for each day there, bottles, sippy cups, favorite stuffed animal, etc. When the trip comes up, I print the lists for each person, tweak it as needed, and check off the stuff as it's packed.
On the day I'm loading the car, or packing last-minute stuff I sit them all down to a movie and threaten anyone who interrupts my fragile train of thought! LOL. They each get to pick 5 things to keep themselves entertained (more or less, depending on room) and if they get bored, I get to say something wise (or smart-alecky, depending on who's critiquing) like "Next time you'll want to choose something else..."
Being able to print off the lists is a huge help, so I don't have to re-invent and forget new stuff every time.
But it is still huge. Everytime. And they'll never appreciate how much work it is. Until they are mothers. Case in point: I LOVED camping my whole life. There was nothing better, more fulfilling. The best family vacation ever. Then I became a mother. What was I thinking? Camping is the worst family vacation ever.
My friend with lots of kids takes a trailer behind her car and throws everything in it. She can't stand a crowded car. And she gives assignments - one watches luggage in the airport, others are in charge of specific children, so she doesn't have to go crazy in crowded places. I do that, too. My 5 year old is a runnaway, so I kid-leashed her to the baby's stroller in the airport. I told her that it was so her little brother didn't get lost.

She believes me. I praise her for "keeping good track of him."
And I hail the inventor of the portable DVD player. They are my friend.