If you have a garage, go the the Dollar store and buy the cheapest plastic trash can AND garbage bags that fit it. Make it a HABIT to throw away trash from your car. You could start with making this a Saturday chore. After awhile you start to like it cleaner and you will clean it out more often. I LOVE LOVE LOVE to burn paper so I pick up any paper trash and take it to my burn barrel plastic trash can that is inside the door close to the washer and dryer, where we also clean out the dryer vent. It is lightweight and I can wheel it out to my burn pile.
This makes the cleaning a lot more fun than a chore.
You can also buy a small trash can, aGAIN with bags that fit for aluminum. You don't get much for recycling, but I started doing this before my DD's left home. They swill down lots more soda than me, but I gave them my "cans can", with the label written on the lid, and they deposit aluminum cans AND beer tops in it faithfully, and I take the bags in to recycle when I can do it.
Also, I have a 2 story house and I put things on the stairs under the railings that need to go upstairs. I take them up on my trips to the 2nd floor.
These tasks are 5 minute jobs. I reward myself when I do them and although I don't think I will EVER have as neat as place as @ninnymary , my house and car are a LOT cleaner than they used to be AND I am finding things all of the time that I don't have to buy again.
Back to your original question, I do transport gardening supplies in my car, but when the items are big, I use my truck.
I keep my present car in MUCH cleaner condition than I ever did my previous cars. I used to terribly abuse my cars. I hauled feed, hay, manure, chickens, dogs, you name it. The cars always smelled of all the above mentioned items. I measured a car's worth (to me) by how many pounds of horse feed it could carry. Top weight was 750 pounds before car squatted. LOL I figured it was equal to hauling 4-5 obese people, just in easier to lift 50 pound bags.
I had a 2003 Ford Focus wagon (I totaled it on 610 loop in Houston ) It easily carried 500 pounds, squatted at 750. I had a 2007 Dodge Calibur, it carried 750 with only a slight squat. Employees at the feed store thought I was nuts.......WTH, I am nuts.
Mom will load her car full of anything needed if she can fit it in there. she's also very neat and will clean it right back out each time. she even puts down blankets and sheets to catch things.
my car i use to haul some things, but i have to keep it nearly empty because the mice will make it a mess otherwise. at the moment i have the back seats and lining from the trunk removed so i can keep an eye on everything. probably going to sell it... *sigh*
we now can borrow one of my brother's trucks for hauling loads of wood chips, boards, bricks, etc. so that helps greatly, but it takes us an hour round trip to pick it up so we only use it once in a while.
NEAT? Hardly But still much better than hubbies truck. Anytime I Want to go someplace with him I have to toss a bunch of trash and move tools, etc... to get in. I do haul everything in the van. Usually in need of vacuuming. Dirt, plant bits etc... boxes of stuff I need to drop off at good will, etc...