When washing a car you are, at the risk of stating the obvious, removing dirt from the car. While this may sound simple it is actually difficult to effectively remove the high quantities of dirt and debris picked up from day to day use without damaging the paint.
Lets consider first the wash medium (what we will wash with). I never use a sponge on any paintwork because a sponge has a flat surface and is likely to trap dirt and pull it across the surface of the paint. This then damages the paint further with lots of small scratches from the dirt particles being forced across the paint.
Instead I would suggest using a microfiber medium (usually a mitt or cloth) or, even better, a woolen mitt. The microfiber or woolen surface is not flat and therefore has room for the dirt to be pulled off the paint and up into the pile of the medium. This reduces the scope of the dirt to be exposed to the paint again.
A proper rinse of the wash mitt in the rinse bucket (see later) will then shake these particles out and permanently off the car while a sponge may not pick them off the car at all instead pushing them around it.
I suggest having a couple of wash mitts and a good selection of microfiber towels. The towels in particular are used for numerous tasks on the car so I always keep about 20-30 available.
Note when you receive your medium I suggest checking them for labels and tough edges. I remove these immediately to prevent them generating their own scratches!