It's good to clean your windows on the inside and out every once in a while, but ugly streaks can make it seem like you never washed them at all. You can reduce your chances of getting streak lines if you avoid washing your windows in the hot sun.
Photo by Andrew Gustar.
Spraying down the outside of your windows with some soap or detergent can help get rid of any dirt buildup, but Ashley Poskin at Apartment Therapy suggests you save the window cleaning products for when the windows are in the shade and cooled off. The sun can heat up the glass, making streaks a lot more likely. For the best results, you can use a squeegee and overlap your strokes, but if you don't have one of those, some newspaper will do the trick too.
How to Wash a Window [Apartment Therapy]