Recently I painted my apartment and found it very frustrating due to my popcorn ceilings; I couldn't get a straight line across the top of the walls due to the textured ceiling.
Go figure that I would come across some useful advice after the fact that I have finished painting, but I thought I would share it to keep anyone who is painting their home that has textured ceilings from experiencing the same frustration as I did.
Here's the simple trick:
Before you paint, drag a narrow flat-head screwdriver lightly along the ceiling. You'll get a clean paint line and no one will ever notice that the bumps are missing.
That's it! Simple as that. No go ahead and paint without the annoyance that I experienced and learn from my mistakes. I can guarantee you that I will be using this trick from now on.