Cloudy skies, clean windows.
Wash windows on a day that is cloudy but not rainy or early in the morning. Working in direct sunlight causes streaks because the cleaning solution dries before you can wipe it off.
Avoid smears.
When you're done, you might notice a few drops of water at the edge of the window. Leave them alone. They will disappear as they dry, but if you try to wipe them up, they'll just smear.
Paint removers.
Paint left on a window can be removed with nail polish remover or softened with turpentine. Then scrape with a plastic credit card or a special paint scraper. Don't use nylon pads or steel wool as they might scratch the glass.
Frames first.
The best time to vacuum window frames and sills is before you wash your windows. Use the brush attachment of your vacuum cleaner to remove dust, soot, cobwebs and dead insects.
No crouching tigers.
When cleaning the outsides of double-hung windows, don't take chances sitting on windowsills. Instead, raise and lower both sashes so you can clean each outside surface from the inside.
Car tip:
Air pollution and the silicones applied in many car washes may produce a haze or an invisible coating that causes wipers to streak and "squeak" and can't be removed by washer fluid. Spray on
Windex® Original Glass Cleaner, safe for even tinted glass.