Several factors contribute to the value of a used car, including its physical condition, its make and model and the location in which it is sold. Other factors include the mileage on the car and whether or not the manufacturer continues to make cars in that line.
The biggest contributor to a used car's price is its condition, as a well maintained car is always worth more than one that requires any work, even if it is minimal. One can increase the value of her used car by tending to any mechanical or cosmetic issues, such as replacing faulty transmissions or fixing dents in the exterior. The specific features of a car also contribute to its price, as some features are more desirable than others. Pricing guides also consider the seller's location, as come regions have a higher demand for specific cars than others.
Another major factor is the car's mileage. The more a car is driven, the more wear it endures and thus the less reliable its parts become. There is little one can do to compensate for extensive mileage, even on classic cars. The best thing to do is perform regular maintenance on the car to keep it in as good of a condition as possible.