If you're painting the ceiling and the walls - then paint the ceiling and go down the wall for about 2.5cm all round. Cut in the wall colour using about 5cm brush (the longer the bristle the better), this way if you sway downwards a little the ceiling colour may hide the mistake, if you sway up onto the ceiling just touch up the ceiling. Practice makes perfect. Happy painting.
I have used a sponge pad with very small rollers on one edge that you can roll along the ceiling. The pad is about 3 x 5 inches. Its brilliant but very time consuming. I'm a bit of a perfectionist so it was worth it.
Loads of people comment on how straight and neat my edges were!!
I don't know what it is called though - I bought it in a paint shop in Waterford for about €5.
With painting the more you do it the better you get like most things-you eventually develop a steady hand.Some years ago I had this problem and I bought a thing in Woodies called a Paint Guard.Its basically a strip of plastic about 18" long which you hold into the angle-wall and ceiling and paint along the underside.It works but just make sure you keep cleaning excess paint off it.Woodies used to have them in the paint accessories area with brushes etc.
nobody has mentioned it bu you should have a ladder tall enough so that you can see the top of the brush Ie you head should almost touch the ceiling. i always use a 3 to 4" brush , because it holds more paint and can be controlled easier as the brisles are softer