I've been trying several ways to keep my mini-van organized. Keep in mind I have two kids aged 7 and 11, drive a carpool and am constantly picking up give-away materials for their school! So the clutter comes whether I want it to or not. The trick is, I know how to keep it at bay!
One thing that works for me is to keep a medium-large container in the back to hold things I will need as I drive around on errands. I'll put bags of things to return to stores in it, folded re-usable sacks, items I need to give to a friend, or books going back to the library. It is all contained. If I ever need to move the rear seats around, I can just pull the whole container out and put it back in - and the contents remain neat and orderly. My container of choice is a woven basket-bag that I bought from a local vendor. It has long stiff handles, a box-like bottom and looks lovely. Many stores sell a nylon trunk organizer system that would work too. My mother uses a plastic crate for her trunk organizer.
Do you ever have loose papers, coupons or receipts floating around the car? I sure do. My favorite organizing tip for the car is to keep a clear plastic envelope, letter sized, to slip next to your seat. I use it to store all those loose papers. For this to truly work, you need to periodically empty the items that go into the house or sort through expired coupons etc. If you actively use the folder, this won't ever be a problem. I put papers to take to a meeting, coupons for a store I know I'll be dropping by at some point, even gift cards for my favorite drive through restaurant.
I also keep certain items in the car always. This would be certain sports gear that my child uses at least 3 times a week. She never uses it outside of class, it doesn't need maintenance, and we always drive the same vehicle to and from class. So that gear stays in the trunk. The uniform that needs washing always comes inside!
A very important rule helps me too: I insist that my passengers take their own belongings out of the car! If they brought too much for one trip, they must return for the rest.
Keep a small bag for garbage. Empty the bag at least weekly. It can be tucked into a door pocket, under the front seat, but always out of the way of feet! I recommend a bag no larger than a bread bag. You can also keep a small whisk broom in a door pocket (or your container in the back!) to quickly sweep out crumbs or sand from seats and floor mats. A little flick of the broom does wonders.
Just like staying organized in your home, you can keep clutter down by taking one minute before you get out at home. Look around, gather the little things that snuck in, take anything extra out of your car. You'll love the way your car works for you on the inside!