1. Wash the wheels first. This prevents you from splattering cleaners, dirt and brake dust on already cleaned panels.
2.Make sure your cars finish is cool and work in the shade if at all possible. A hot surface causes the wash and rinse water to evaporate too quickly, increasing the likelihood of water spotting. One trick I`ve learned is to park on a slight incline. This allows the rinse water to run off moldings, trim, and recessed areas better. Start by thoroughly wetting the cars finish with a medium spray of water to remove loose grit and surface dirt.
3.Use a car wash shampoo specifically formulated for automotive use. Look for a shampoo containing surface conditioners. These conditioner act as a lubricant. Sand and abrasive grit slide off in the rinse water without scratching the surface. These shampoos cost a little more but are worth every penny if you own a dark color car. Meguiar`s NXT Conditioning Shampoo are both highly recommended if you own a black, red or dark color car. Follow the directions on the bottle for the proper mix ratio. Using too much soap is wasteful and may leave a slight soap residue on the surface.
4.I prefer a Ultra Microfiber wash mitt as microfibers reduces scratches by shielding dust and micro contaminants under its strand. The "picking up" effect of microfibers also makes washing more productive and efficient. Wearing Ultramicrofibers wash mitt as a glove also allow easy reach while washing micro areas like door handles and bumper curves. Natural sponges have a large number of fine filaments that draw dirt and grime away from the surface being cleaned into their internal cell structure. Synthetic (flat sided) sponges trap dirt, grit and grime on the surface which can cause fine scratches in the paint. If you use a natural sea sponge, use the outer outside for washing, not the harder outside and always rinse new sponges thoroughly to remove any remaining sand or shell particles. A natural sea sponge should last 12 to 18 months of normal use.
5. Once you have your bucket, soap and hose, start washing from the top down and rinse the car often. Frequent rinsing is especially important if you are using a wash containing natural oils. While these oils cushion the paint and minimize abrasion they are heavier than water and can leave a film if allowed to dry on the car. I use a final rinse of free flowing water(nozzle off the hose) allowing the water to sheet off the car.
The above was abstracted from the link
http://diygurus.com/catalog/pwashing.php