The first factor that any car appraiser looks at when deciding the value of a used car is the condition of the car's exterior.  Rusty spots, untreated dents and dings, and built-up road muck will drop a car's value faster than anything else.

If you've been manic about your paint since the first day you brought the car home, you have little to worry about.  Faithful washing with a soft mitt and pH balanced soap, and frequent waxing with a UV-blocking car wax goes a long way over the life of a car. 

Obviously before you trade a car in, you want to clean it up as best you can.  If you finish washing your car and find that the finish still feels rough to the touch, it's because dirt and grime has worked its way into the clear coat, which is the thin layer of clear paint that gives a car's finish its true shine.  Luckily, there's such a thing as detailer's clay that will get this dirt out that normal washing can't remove.  This will make your finish as smooth as the day you bought it, and may even return some of that lost shine.

Follow the directions on the box and you'll be good to go!