A dog that's fine on short trips to the park can still have a rough time on a three-hour drive. A bit of preparation makes the difference between a stressed, carsick passenger and one that sleeps the whole way.
Secure them properly, every trip. A dog loose in the back seat is a genuine safety risk in a sudden stop, both to the dog and to everyone else in the car. A car harness that clips into the seatbelt system, or a crate secured in the boot, should be non-negotiable - treat it the same way you'd treat a child's car seat.
Protect the seat before you leave, not after. A waterproof, hammock-style seat cover stops mud, fur, and the occasional accident from reaching your upholstery, and it's far easier to hose off or wash than trying to get dog hair out of fabric seats after the fact.
Pack water before you pack anything else. Dogs can get dehydrated faster than people realise on a warm drive, and servo water fountains aren't built for them. A collapsible travel bowl takes up almost no space and means you can offer water at every stop without digging through bags for a proper bowl.
Stop earlier than you think you need to. Build in a break roughly every two hours for a toilet stop, a stretch, and a drink, even if your dog seems settled - it's much easier to prevent restlessness than to deal with it once it's already started in a moving car.
None of this needs to be expensive or complicated. A harness clip, a seat cover, and a collapsible bowl cover most of what actually goes wrong on a road trip with a dog - the rest is just common sense and a few extra stops along the way.