Make sure to regularly change the oil to make sure the car is running properly. Take care of your car, and it will return the favor (and by favor, we mean money).
Keep every report or receipt you get (oil changes, insurance claims, etc.) that involves your car. Having these documents will show a potential buyer the history of the car (they will feel more secure about buying if they know what its been through).
More miles on your car will equal a lower value. Be mindful of how many miles you put on the car (13,500 per year is average) because it will pay off when you are looking to upgrade.
Spending thousands on modifications won't result in getting more money. In fact, usually they decrease the value, so be cautious when purchasing those killer exhaust pipes for your corvette.
Sure, you like your car to be an extension of your personality, but there's a very limited market for rainbow Beetles. Stick to basic colors to ensure that your car will attract a potential buyer.
Loose knobs or broken buttons are little things that can be easily fixed. If a buyer sees that they will have to do the work, you could be losing out on money!
Having crystal-clear lights (instead of dull, hazy ones) will help the car's overall appearance.
You might spend $100 or so to repair those tiny dents, but you can easily ask for a higher selling price when the car is dent-free.
You don't have to go out and buy the $300 per tire brands, but some new rubber will allow you to command a higher price.
Avoid all eating, drinking and smoking in your car. These smells are nearly impossible to get rid of and cause your overall car value to plummet.
Make sure there isn't any pet-hair anywhere or rocks all over the floorboard. A car with a spotless interior will help your wallet get bigger.
You should wax your car at least once every three months. This will keep it looking newer for longer and help prevent weather damage. First impressions are key to selling a used car and waxing makes a huge difference.
Shop your car around to get estimates before selling. This way you can get an idea of the market for your car (possibly increasing the value if demand is high).
Whether you have disc brakes or drum brakes, a smooth stop will surely increase the value of your ride.