Make a half and half mixture of vinegar and Dawn (the basic blue stuff). It mixes better if you warm it up a little bit. I've seen suggestions that you should add the Dawn to warm vinegar, but I find it works better if I put about a quarter cup of vinegar in my glass measuring cup, add a quarter cup of Dawn, and then microwave the whole thing for about 30 seconds.
Add it to a spray bottle - you don't have to use it while it's still warm, it just mixes better that way.
Spray on the shower, leave it for at least an hour - preferably two or three, but all day is fine, too.
Come back, rinse it off. If it doesn't all come off, give it a quick swipe with a washcloth.
Done. Total time spent on it? About 3 minutes - including microwaving and mixing the solution. (Plus, the Dawn helps with the vinegar smell. Not that it's odorless, but it's not that super-strong vinegar smell, either.)