Keep a crate or box in the trunk for ERRANDS -- items to return to the store, dry cleaning to drop off, library books to take back, etc.
If you like to take care of "to-do's" in the car carry a portable SUPPLY with you -- pens, stapler, paper clips -- whatever you need to do work on the road.
Keep the paperwork in your glove COMPARTMENT organized with a labeled expanding file -- "maintenance, " "registration," "insurance."
Take a minute each day to CLEAN your car out after a long day -- toss out the trash and take important items in the house or office.
If you carry any equipment or supplies back and forth to the car, put your "stuff" in a plastic portable CRATE with handles and a lid.
You can keep your car a great deal neater just by keeping a small TRASH can in the floor of the back seat.
If you use your car for work, consider the many mobile OFFICE products that are available to help you be efficient in your vehicle.
Keep a road EMERGENCY kit in the car -- blanket, flares, flashlight, jumper cables, jack, oil, ice scraper, etc.