More than one-third of car owners (according to the Car Care Council) use damaging non-automotive products when washing their cars-products. Such products typically contain harmful detergents, abrasives and additives.
Almost half of motorists don't ever wax their vehicles. Though twice a year is recommended to continue protecting a car's paint job. Improving trade-in value is important because a car in 'fair' condition can increase its value about 50% if it were clean and well-maintained, according to the Kelley Blue Book. You should still wax your car, even with a Clear-coat finish. Its only as thick as a piece of paper and can become damaged from the cumulative effects of sunlight, UV radiation, acid rain, salt, dirt and air pollution.
Dish detergent should be avoided, though "mild for your hands" they contains harsh chemicals that cut through grease but also strip away the wax finish on your car. And some are hard to rinse off and leave streaks. For best results, a specially-formulated automotive wash is better because it is gentle on the car's finish while still removing dirt & grime.
The Car Care Council has made a couple of recommendations when doing a do-it-yourself car wash: