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Contact lens cleaner. Takes the realty stuck on bugs off and no streaks. Expensive but I always keep the expired left over from my daughter. Works really well
Sprayway aerosol glass cleaner is the best. Go into almost any glass shop and you will see cans of it sitting all around the shop. The wife does art glass fusing and she uses it exclusively. Works great on the inside glass of vehicles.
Remember "Glass Wax" from the old days? (Used it to "frost" windows for Christmas) It has a bit of clay in it, actually polishes the glass. I need to find some, the original isn't sold any more, but Gel Gloss sells nearly the same thing.
Also, use newspaper (black print only) instead of paper towels.
Old army "GI party" method for streak free windows or mirrors definitely works: spray bottle full of white vinegar with 1 or 2 drops Dawn (no more or will streak), use wadded up newspaper pages to scrub and polish dry. Change portion of wad in use and switch to new wad frequently. 6-8 pages to do windshield: 2 turned 3-4 times to scrub each side (in and out) with solution (4 total), 2 to polish each side streak free. 3-4 pages for each side window. Do not clean hot glass or in sun! Also do not rub newspaper against light colored upholstery fabric.
Ammonia cut with water, seems to work fine. Windex is basically ammonia and blue dye. One recipe is Ammonia, rubbing alcohol, cut with water and a drop or two of dish soap.
Newspaper works well for windows. They sure as hell aren't any good for reading.
Easiest way....use windex, but use plain old newspaper to wipe the windows with. The newsprint is a little abrasive on a very small scale and the paper absorbs better that any paper towel. It's best to avoid the pages with color printing.
It might work even better with some of the solutions recommended by the other guys, but the newspaper is the key.