Appearance matters when buying and selling used cars. But few buyers ever make a purchase decision based solely on kicking the tires and walking around the outside of the ride. Their next move is to open the driver's door and peer inside. What they see and smell can make or break a deal.

The condition of your car's interior says a lot about how well you maintain the vehicle. Automotive dealers often spend more time touching up appearance issues than mechanical ones. Some call in the pros, like those from M.A.R.S. International, a franchise appearance-reconditioning company headquartered in Arlington, Texas.

"People have two primary issues with the insides of their cars," says M.A.R.S. President Allan Kelsey. "It either gets soiled and dirty or it starts to smell." Kelsey says that just about anything can damage the car's interior, but frequent causes are using child car seats, smoking, eating while driving and carrying objects like ladders inside the car.

"You can prevent the damage or you can cure it," Kelsey says. Prevention means being more careful, not eating while driving, using floor mats and periodically cleaning the mats and seats. "If prevention isn't doable, then the cure is where we're at," Kelsey says.

Many of the problems Kelsey's operators deal with can be repaired in 30 minutes, often while car owners shop for groceries. But they're professionals with special tools and solutions. Still, he says, consumers can do their own cleaning and preventive "maintenance" to keep car interiors shiny, clean and odor-free.

David Kellis of the Clorox Company, makers of the Armor All products, says they recommend a thorough cleaning "at least twice a month." The company markets wipes you can keep in the car for spot cleaning.

When you're ready to thoroughly clean your car, start by vacuuming.

"We recommend vacuuming interior surfaces before cleaning with our products," Kellis says.

Work from the top down, so that particles you dislodge will fall to the seats and floor for easy pick-up as you vacuum below. Kelsey says one reason carpets deteriorate so quickly is that accumulated dirt at the bottom of the carpet fibers acts as an abrasive and causes fibers to fray and fail. Regular vacuuming and cleaning will help carpet last longer.

"Removable mats are intended to get dirty; they're not that expensive to replace even [if you buy them from] from the dealer," Kelsey says.

To wash removable mats, mix up a fairly strong solution of laundry detergent and water, he says. First, soak the mats with water from the garden hose or another source, then scrub the detergent mix into the mats with a medium bristle brush. Spray off excess soapy water, squeegee out excess water and hang the mats to dry.

When cleaning hard plastic surfaces such as the dashboard, you can use household cleaners, be sure they're safe for the surfaces in your car. Better still, use interior car-care products, which are designed for automotive surfaces. To avoid splashes on the inside of your windshield, be sure to spray the product onto a cloth first then wipe the surface. This also minimizes streaks on the surface you're cleaning.

Spray-on upholstery cleaners can be used for routine cleaning of fabric seats or you can use a steam cleaner on seats and carpets. However, most won't be sufficient when it comes to lifting tough stains. For these, you need to use special cleaning methods or turn to a pro.

For leather seats, use leather cleaner and a medium bristle brush. Kelsey says the brush gets down into cracks, where the real dirt lies.