I have always found this chore a right pain. I use water with a splash of w/u liquid, skimmed of with a squeegy, then buffed up with a dry cloth. It takes best part of a day (lots of windows) but no matter how much elbow grease I use to buff up, when the sun comes round it is still smeary. I now employ one of those guys with a long brush and de-ionised water. For £10 its money well spent. Be careful though. I have been quoted £25 by eastern European types doing exactly the same.