According to Green Living Tips, it is better to take your car to a car wash than to wash it by hand. But surely this depends on how you wash.
I use a set of three microfibre cloths. The first is like a glove and gets the main dirt off with water. The second is a heavy duty cloth that works best when dampened and takes off the remaining dirt. And the third is a chamois-like cloth to bring up the polish of the surface. No detergent is needed. Just use washing up water.