The Right Weather
Never clean windows on a hot, sunny day. The sun will dry the cleaner before you can wipe it off, leaving streaks. Choose a cloudy day instead.
The Best Cleaner
You don't need expensive blue liquid. Mix 2 cups of water, 1/2 cup of white vinegar, and 1/4 cup of rubbing alcohol. The alcohol helps the solution evaporate quickly, preventing streaks.
The Technique
Spray the window thoroughly. Wipe with a clean microfiber cloth or a squeegee. If using a squeegee, wipe the blade with a dry cloth after each pass. Overlap your strokes slightly.
Newspaper Trick
Believe it or not, crumpled black-and-white newspaper is great for polishing windows. The ink acts as a mild abrasive that removes residue without scratching.