Okay, we realize that this may not apply to everyone, not by a long shot. Some have spotless car trunks, many don't care about this anyway, and some live in fantastic urban cities that do not require the ownership of automobiles (lucky ducks!). SO! If you do care, and if you spend an inordinate amount of time in your car like I do, living is Los Angeles, this little Tiny Tidy is for you. So, here we go!

Pick a safe time of the day to do this, if you have to park outside, or in carport, thats' relatively "quiet." If you have a garage, lucky you. you can do this at 2 am, if you wish!
Put a few garbage bags or old blanket on the ground.
Take everything OUT of the trunk. (Believe me, this is painful for me too, I feel ya.)
Put everything from the inside the trunk into your living room, and put it back in its proper place. Maybe it's old clothes and shoes meant to be donated? That stuff goes in your front seat, to be taken to the Goodwill or drop-off bin stat. Are there a few boxes of left over Christmas things to be returned? Work supplies? What? (If you need to put it back in the trunk because some stuff you actually use daily or weekly, that's okay, we'll do that next)
Depending on whether or not you have a Dustbuster, vacuum up the trunk or, are feeling quite vigorous and sans personal, hand-held vacuum, take the car to a instant car wash place and vacuum out the trunk there.
Drop off anything that needs to be donated on the way there or back. (DO IT! :)
Go back to the living room. Now, what really needs to be in that trunk? Ideally, we are just talking about things like emergency kits, water, extra reusable shopping bags, and maybe some postage supplies, PLUS- whatever you do need for work or life (supplies and sports equipment, sports, kids etc.) Decide. Be ruthless!
Organize this stuff now. Just go ahead and do it, it will take fifteen minutes, tops, I promise. Hopefully, you have stashed away, recycled or donated the excess stuff that was being hoarded there.
If you are so inclined, and definitely do have stuff that NEEDS to stay in the trunk of your car, grab ONE small bin, or a few pretty, square baskets that you wouldn't mind utilizing.
Bring everything back to the car's trunk outside (or in garage) along with the baskets or bin, and start to organize. Do it fast- please do NOT overthink it. This is not a Martha Stewart marathon. Just a good, old fashioned tiny Tidy to make your commutes more pleasant and enjoyable.