Mitts are preferred over sponges. Sponges have a harder time of dispelling debris and you have to push harder to make full contact with a sponge so it's more likely to scratch.
Another point is "how" you use the mitt. I always lightly wipe in a fore-aft direction with the mitt in overlapping passes. Don't use big circular motions. Any defects will show up more that way.
Although I don't use the two bucket method, it is sound advise. I have the luxury of having running water readily available and use it constantly to clean my mitts.
Another trick I use is with the flowing water. I hold the hose (plastic end, not metal, for safety) against one edge of the mitt as I draw the mitt across the car. The water is constantly flowing through the mitt and taking the dirt and debris with it. I am never pushing dirt along the body. This water flow constantly lubricates and cleans the mitt.
I also always use two mitts. I wash the roof, hood, trunk and top half of the doors with my best mitt. It always stays very clean.
I wash the bottom half of the car (below the beltline and lower bumbers) with a second mitt. I keep them seperate at all times. There is less chance of me picking up dirt and scratching the more noticable surfaces that way. Sounds fanatical but it's really no big deal.
