I just removed popcorn ceiling last week. Here are two helpful hints.

Tip 1.) I used a 1/2" nap roller and submerged into hot water bucket. I let it drain slightly, then begin to roll it onto the ceiling in about a 5X5 area. I wet the roller again and then went over it a second time. I moved on to do another 5X5 area, twice. Then I went back with a semi brisk kitchen broom and begin to just sweep the ceiling clean. I continued this wetting ahead and allowing the ceiling to absorb the water process until finished. It came off onto the plastic, I placed covering the floor, for easy clean-up really nicely. The broom preventing any gouges with a putty knife.

Tip 2) If following up with a blown-on knock down finish, make for sure you get all the popcorn ceiling cleaned up really well. Otherwise you may not see the balls, but when you go to knock down the texture with your trowel, you will drag these little balls and leave lines or grooves in your nicely textured walls or ceilings. It took me awhile to figure out what was going on. I kept cleaning my trowel and again it would happen.