What happens to the exterior of your car (staying clean versus getting dirty) is pretty much determined by the weather in your area, so it’s out of your control. But what happens to the interior is very much something you can have some control over, since it’s your domain.
What do you think is the number one thing that makes a messy car? If you said, “food and drinks,” you’re right. How many times have you accidentally dropped some French fries in between seats, or left a couple wrappers (for days or weeks at a time) here and there in your car? There are two schools of thought on dealing with food and drinks in your vehicle. One, you can ban them from even being inside your car, such that you and/or your guests never eat or drink anything in there, other than, perhaps, water. Two, you can choose to eat and drink in the car but have a designated trash bag/container conveniently with you at all times. This way, you have one spot to throw away junk, rather than just tossing it anywhere.
As for the surfaces of your car, they do get dusty over time. Guess what you could use to wipe them down occasionally? Baby wipes work well for this very thing. You can keep a pack in your glove compartment.
If you’re the type of person who always seems to have junk on the floors, from pebbles to Cheerios, consider using a Shop-Vac every now and then to vacuum the flooring/mats/rugs. You can keep this vacuum in your garage and use it as needed.
When you’ve got grime stuck in tight spaces in your car, try wetting a Q-tip with water to get/pull the grime out. This works well around your dashboard and anywhere there are grooves that accumulate stuff over time.
Finally, a great way to keep a car clean is to make sure you don’t let it become the place where you just throw stuff and leave it for months on end. Every time you put something new into the car, take something old out, if you don’t want to have a messy car overflowing with items.