Perfect paint lines: tape 1/4 in above and below ceiling line. Caulk in between, smooth out with finger. Carefully remove tape before caulk dries. Perfect line, impossible to tell it's really caulking!
Lap Your Cut-In Onto the Walls - If you're planning to paint the walls too, lap the paint onto the walls a little bit. Then when you paint the walls, you can err on the side of leaving a little ceiling color showing when you cut in and it won't be noticeable. Some painters like to skip this cutting-in step and save time by mashing the roller into the corner instead, but this method is sloppy, builds up excess paint in the corner and can leave runs or a thick paint line on the wall.