The professionals (http://www.rbcclean.com/)who cleaned our windows last summer,shared this amazing tip with us. They suggested the use of newspaper instead of cloth or paper towel. Spray a mixture of vinegar and water (or any other window cleaning liquids) on to the glass and then use newspaper to clean the glass (Avoid the window frame). The ink and the paper act as an abrasive and leaves us a clean window. Hope this is helpful.