Don’t allow eating in the car. Crumbs are a nightmare to get out of nooks and crannies, even with an industrial strength vacuum.
Be cautious about liquids. Sometimes hydration is important when driving but ask people to use a cup rather than swig out of bottles in order to reduce spillage risks.
Only allow water to be consumed. If someone drops a cup or bottle of water in your car, then it’s a minor nuisance. If they drop a cup of sticky sweet red fruit juice (etc.) then it’s going to be a much bigger staining and removal challenge.
Don’t allow pets in your car. If you’ve ever spent hours trying to get pet hair off your seats and carpets, then the sense of this recommendation will be self-evident.
Keep a sick bag at hand when travelling with younger children. A child can be fine one second and in the blink of an eye, they’ll start vomiting for no obvious reason. Your notice is likely to be minimal and pulling up isn’t always possible in time. A receptacle of some sort could save your upholstery and a very unpleasant cleaning job afterwards!
If you can, choose leather or faux-leather upholstery. It’s much easier to clean and often harder to stain. In fairness, some modern fabric seats are good too but still probably not as ‘wipe-able’ as leather or simulated leather.
Keep an emergency seat cover in the car. This is invaluable when your kids have come back covered head-to-foot in mud and you can’t exactly ask them to sit undressed in the car for the drive back home.