The method which works best on glass shower doors is to mix vinegar, baking soda, dishwashing liquid and a little water. Spray on liberally, let it soak for a bit, then use a soft scrubber in circular motions to loosen up the build up, and rinse. Cheap , Non-Toxic, and the vinegar helps kill and control mold and mildew!:0). If the stains are extremely set on and this doesn't work, buy some 'Barkeepers Friend'(Oxalic Acid),cleanser at any grocery or hardware store, apply to damp glass, let sit a bit and scrub away. Make sure to wear gloves and do not ingest or get it into your eyes. Good luck!:0).
Magic Eraser sponges. They get everything off!!! I had water stains from a plant sitting on a plate and the water leaking onto the plate for months. A lot of elbow grease scrubbing it did nothing. A Magic Eraser sponge got it off with very little effort. They also get crayon off of walls, scuff marks off floors, and even marks on cars from fender benders.
If you know an AMWAY distributor please purchase from them window cleaner, just spray and wipe as simple as that. If you don't know any just write to Amway, they will get a distributor to contact you :))
I'm a physicist by trade but a chemist by hobby, so I know that this will work. This is gonna sound crazy, but *alcohol* (specifically, alcoholic beverages with content > 100 proof) will totally cure this problem. (1) Alcohol dissolves alot of stuff, (2) alcohol evaporates faster than water, and leaves very few traces behind, (3) whatever is left behind after using alcohol can easily be wiped off with a cloth. Here's what you do: clean the window using hot water and soapy wash, then pour alcohol over the window. Don't be cheap about it, either, pour the stuff like it's water. Vodka is the best next to grain alcohol, and 151 rum is also available but at considerable expense. It will dry in seconds, then wipe. Job DONE. Good luck, my friend!