Start a cycle and stick to it. Any time you bring something into the car, get in the habit of taking it out when you exit your vehicle. It's about your routines, not about the stuff.
Visor organization. Keep a heavy-duty rubber band on the driver's side visor to hold parking tickets, directions, mileage logs and any other papers relevant to the day's travels.
Your new-car smell. Keep your car smelling sweet by placing fabric softener sheets under the seats or filling the ashtray with baking soda.
Keep a small cooler in the car. Keeping a cooler in the rear or trunk to carry refrigerated and frozen groceries helps you run more errands after shopping (and keeps them from disappearing in your trunk, too!).
Spare clothes. Keep a small travel bag with a change of clothes in the trunk, just in case.
Maps. Protect them from travel spills by storing them in zipper-lock plastic bags.
A versatile tool. An old, clean shower curtain stored in the trunk comes in handy as a cover for a summer picnic table, a drop cloth for changing a tire or a protective tarp.
Prepare for trips before you leave home. Check traffic and weather reports before you head out and make sure your emergency kit is stocked. Make important phone calls before you get in your car to avoid talking on the phone while driving.