OHW

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Since they overlap each other, you just need to find the last one nailed up, and take it out first, find the pattern and follow it. The nails should come out pretty easily, don't worry about bending the edges, you will straighten them later when you reinstall it. Use a small flat prybar and nail puller.
Take lead paint precautions; and there will probably be a lot of filth sitting on the topside of the old tin.
Casey

No personal experience, but here are some tidbits from eHow, doityourself.com and finishing.com, just to put a polish on Casey's expert instructions:

"Rolled tin ceiling is unwieldy and has sharp edges. It can cut and gouge surfaces."

Put painters' tape on any wood moldings that might get damaged. Use multiple layers if you think it best. [Unless the building is about to be torn down and nobody is taking the moldings. ]

Prep should include turning the electricity off at the breaker box, and then removing any light fixtures in the way.

Start with the cornice (if any), which typically came in 48-inch lengths.

Slip a bit of lath or cardboard or something underneath the pry bar to protect the wall (if necessary).

Good luck!
MelissaKD

Quickly? Well that could be the toughest. It will likely take a bit of time, but if you are patient it will pay off. Definitely get a slim prybar to work on the edges as Casey says.
Another handy tool would be a pair of side cutters or wire cutters. Once a nail is a bit loose, use the plyers to pull the nail so you don't have to pull on the ceiling panel. The tapered edge slips nicely under the nail head.

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I hope the pews are gone before you start! Renting a scaffold would likely be a good investment. I'm guessing the ceiling is pretty high?