Kelley Blue Book, NADA, Edmunds... You can go to a dozen websites and get a dozen different numbers....

Resale value mixes a car's age, mileage, condition, and equipment levels with other factors like popularity or supply.

Brand. It's the painful truth for fans of some popular nameplates known less for reliability. The first name of a vehicle can have a tremendous effect on its resale value, regardless of the condition or quality of the vehicle in question. Certain vehicles are known for rapidly shrinking value. Among the best? Subaru, Jeep, and Toyota rank atop the mass-market brands according to residual value forecasting company ALG. These brands can retain more than 60 percent of their value after the first three years of ownership, while some vehicles from those other brands can fall to as little as 35 percent of their new value in the same time.

Drivetrains. Some buyers may like to shift their own gears, but most buyers don't.... but sometimes the manual transmission can be more valuable than the automatic, especially on a sporty model.

Then there's the choice between all-wheel drive and four-wheel drive. Except on rear-wheel-drive sports cars, all-wheel drive tends to add a bit more.

Paint color. It can bring you down in terms of value. If you don't get white, silver, or black on a traditional Camry you're risking a slower sale. Put a Sunset Orange Metallic hue on and you could be looking at not selling the vehicle at all, for lack of interested buyers.

Technology. In-car technology isn't always your friend. Early adopters pay for the privilege of having the latest technology in their vehicle. Items with limited appeal, like DVD & NAV, may not earn back any of their up-front cost if it's already outdated.

Performance parts and aftermarket accessories. Those 19-inch chromed wheels or high-end stereo? Odds are that what you paid extra will have little to no value to the used buyer.

Above all, steer clear of add-ons that alter powertrain or safety equipment. A $200 engine chip could bring you more power, but nullify factory coverage. Be cautious of anything that might void a warranty or prevent a vehicle from certified pre-owned qualification.

Exterior condition. If you've kept your car in good cosmetic condition, there's not much to worry about. If you never washed it you could have problems. 

Peel off all your political stickers, college logos, and fish logos before you try to sell. Bland, inoffensive, and shiny sheet metal are your best allies.

Interior condition. A clean, well-kept car wins from first glance, or smell. If you're a smoker, a pet owner, or if you eat and drink in the car, you'll want to address those issues. An inexpensive set of new rubber floor mats can look much better than stained, torn, and worn factory mats.

Mechanical condition. Service records can be valuable. The more guesswork you can take out of a car's history, the higher the resale value you can command. Prove it's in great running condition and been loved.

Overall mileage. You can have a perfect car on the block--but with 300,000 miles, few buyers will be interested. Fewer miles usually translates directly into a higher resale value.