One of the first things buyers of used cars look at is the mileage on a car. Excessive miles lowers your bargaining power. Also, the less your car is out on the road, the less chance there is that you will build up excess wear and tear. Long road trips should be done in a rental car. The cost of rental will probably be less than your car's depreciated value from the extra miles. So if you want to maintain your car's value, drive it less!
Avoid extreme start-and-stop driving, over-towing, and excessive acceleration and speeding. Also try to avoid driving where road conditions are poor.
Car Service and Record Keeping
Service your car regularly as recommended by the owner's manual. This means regular oil changes, filter changes, fluid changes, and careful monitoring of brakes and the engine belt. Keep receipts and records of all servicing your car has undergone to give a good demonstration of your car's maintained value to potential buyers.
Keep the Car Clean
Looks are important, both on the inside and out. Detail work, vacuuming the interior and wax jobs on a consistent basis can go a long ways in maintaining car value over time. Be sure to clean off salt deposits that can lead to rust properly and quickly. Rustproof the underside of your car to prevent the metal components beneath your car from rusting. No one wants to buy a car that looks like diseased metal.
Eating, Drinking, Smoking
Basically the 3 Don'ts. Avoid the possibility of spillage or stains. Food also leaves residual odors that stink up the interior, as does smoking. Maintaining car value does involve concessions on your part.
Shelter Your Car
Cars don't do much better than humans in extremely cold or hot weather. If you have a garage, store it. Storing in a climate controlled environment prevents conditions that decrease the value of your car, such as rust. Too much sun can fade the paint job or cause sun damage; extremely cold weather and environmental conditions can mess with your car's internal engine components. If you live in a place where flooding is common, then store your car in a place where the water won't reach. Flooded vehicles are far more likely to rust in the worst places possible.
Prevent it any way possible. Asides from the obvious visual degradation, it can lead to serious problems with the car's functioning.