I find that giving kids a place to put things is the easiest way to get them to organize on their own. I recognized that my daughter had a place for everything in my car except her backpack which ended up on the floorboard where it was inevitably stepped on or forgotten.

My solution is this hook. It simply grabs one of the posts of the headrest in front of the car seat. It keeps bags off the floor and gives the little people a roll in keeping the car clean.