Mine is most often a mess (of course, so is my house as others have said!). I was going to keep mine spotless since my husband's was usually messier, but we end up transporting the kids equally. Some things I started doing that have helped me a bit include:
Keeping a bottle of cleaner and a few rags or paper towels in the car (Armor-All is my favorite). I can wipe off the dash and such when I'm stuck in traffic or at a train.
I keep extra grocery bags in stored for trash. I don't always get it emptied but it all goes in the bag.
I try to do a deeper cleaning every few months, including vacuuming, when my kids are playing outside. I bring everything in, clear out trash, vacuum, and wash down the gross things inside. If I have to, I strap my kids into their car seats and let them talk to me while I do it (otherwise they wander off where I can't see them).
One of my biggest problems with keeping my car clean are leaving half the coffee mugs from my house in my car (I don't own a travel mug) because I never remember to bring my mug back in when I get home from work. I also tend to leave junk from the work day on my seat, or in the winter my heels on the passenger side while I wear my boots in, or in the summer my sneakers are on the floor with my sweats from my lunch break exercise. I think I will also try to keep my car cleaner this winter since my commute keeps taking longer. :)