Spray a stain repellent (such as Scotch Guard) on your carpet and upholstery before you go. This will make accidents easier to clean up along the way.
Keep a large trash bag in the car. Instruct everyone to put trash directly into the bag. Bring extras so when one gets filled you can dump it and start another.
Clean out the car at every stop. Get rid of trash, empty ashtrays and check under the seats for stray French fries or juice bottles - they'll start to smell if not picked up.
Make frequent stops. If a small child wets his or her pants in the back seat, you will have an upholstery odor problem that will be difficult to fix.
Keep smoking to a minimum. Nothing sours the air faster than cigarette smoke, and carpets and upholstery will absorb the smell.
Eat outside of the car whenever possible. Spills and odors are inevitable if everyone's eating lunch in the car; stopping at a picnic or rest area to eat will help eliminate accidents.
Consider using an air freshener. These come in spray bottles, or are designed to hang from your rearview mirror. Citrus smells are popular - and effective in eliminating minor odors - but these days you can get them in just about any fragrance.
Use aromatherapy to keep the car smelling fresh and to help you along the way. Diffusers (like DriveTime Aromatherapy Air Freshener, which you can find online) can be plugged into the car's cigarette lighter and filled with aromatherapy oils.
Have the car cleaned thoroughly when you return from your trip. This includes getting the carpet and upholstery vacuumed and shampooed, and treating any residual stains and odors.
