I strongly recommend this
http://www.californiacardusters.com/...carduster.html
Run over the car with the duster first to remove any sand or potentially abrasive debris. Then just use a damp towel with warm water followed by a dry towel.
Using that method, you will never harm your paint. The dusters are awesome too if you live in an area that collects a lot of pollen or dust. Good luck.