How do I get those stubborn water spots off my glass shower door?

Comet cleanser and a scrubbing brush

I use a bunch of used fabric softener sheets saturated with vinegar. I also have used canola cooking spray oil and a green kitchen scrubby. The spray also leaves a light film that prevents buildup.

Cut a grapefruit in half, cover with kosher salt and use as a scrubber.

Lysol Power Scum cleaner in the blue trigger spray bottle with a Magic Eraser or a scrub brush.

Wet & forget shower for a slower, no scrub solution. You can use 2x a week to accelerate the cleaning process then once a week to keep it clean.

Rub spots with lemon juice on a sponge....wet dryer sheets..or soaked paper towels in white vinegar and apply to spots for 15 mins then buff clean. Keep a squeegee in shower and use on doors when you are finished and/or apply Rain-X glass treatment

Sorry to disagree with Mousy, but you shouldn't clean glass with anything abrasive like Comet. It will scratch the glass and leave you with permanently cloudy glass. If you are referring to soap scum, I have successfully used both ScrubFree and Kaboom. And yes, keep a squeegee in the shower.