cover the floor with drop cloths, use a garden sprayer filled with hot water. Work in small 2ft square or so, spray with hot water, let it set a minute then use a putty knife and scape it off. It's fairly easy as long as it hasn't been painted.
I will second Yvettas' comment. I use a backpack garden sprayer but I use cold water usually. The biggest problem people make is they are afraid to get the ceiling really wet and that's what it takes to get a popcorn ceiling to separate from sheetrock. Once the popcorn is removed you will need to get the ceiling sheetrock finished as they usually only taped the seams for popcorn ceilings.
Sherwyn williams has a scraper that you attach a plastic garbage bag to and scrap the ceiling right into the bag BUT still have a shop vac ready! to clean up when done. I believe home depot or lowes should to, SW is were i got mine.
Make sure that you test the material before you start scraping. Many popcorn ceilings contain asbestos.
Check out this thread.
We concur with Yvetta and all above, we just scraped one ourselves. It's VERY messy so protect everything underneath very well. You can also work with a wallpaper steamer, get the popcorn texturing good and wet before scraping.
I have only tested 4 maybe 5 ceilings that WERE contaminated or "hot", But the timeline for the "popcorn" (acoustic is the more common term ) look and asbestos go hand in hand so be cautious. Assuming it's not ACM or asbestos containing material, use a hudson sprayer with LOTS of water and buy PLENTY of visqueen and some tarps. You will get a knack for the right amount of time to let it saturate and when to start scrapping it with a taping knife. I would suggest that you try to let the existing paper tape that is on the joints remain as undisturbed as possible. If you are careful enough you will be able to start the re-taping right on top of the old tape. Make sure to cut out any loose or blistered tape areas. A few big sponges come in handy for wetting little localized spots. Try to use only sponges and water and avoid any sanding as there is no such thing as "safe" dust, only bad and REALLY bad dust.
Instead of removing why not cover it? I found this a couple of days ago while I was browsing Hometalk... http://www.hometalk.com/search?filter=ceilings#1569100/popcorn-ceiling-makeover-low-budget-big-impact?&_suid=137052603836805147305574623879 There are a few other ideas... such as using bead board to cover it.
check out this article I wrote
I've removed 2 ceilings, so far. Yes to all of the above. But I would strongly suggest using a plastic putty knife. It's too easy to gouge the sheetrock with a metal one. It's so darned easy to remove the texture. Make sure to sand the tapings before you paint. I primed and then 2 coats of paint. Afterwards, I painted on a coat of opalescence and kind of made swirls. It masks the roughness of the unplastered ceiling. And it's Purdy.