Good advice in this thread, except for that thing with wheels. My technique is pretty much as described. Someone mentioned that there isn't a true corner between the ceiling and wall -- it's actually slightly rounded.
Where I differ from the advice given is that I prefer bringing the wall paint to the top of that curve because it's less noticeable than having white ceiling paint on the wall. It's a line of sight thing -- during normal activities in a house, walls [and their edges] are more noticeable. Edges of ceilings aren't noticeable unless you're pretty much up against the wall.
Of course, it depends on the wall color. If there's a big contrast between wall color and ceiling color, ceiling paint down onto the wall will be more noticeable than vice versa. If the colors are really close, it doesn't matter much how straight the line is.
One thing not yet mentioned -- if you get the wall paint too far onto the ceiling, you can touch up the ceiling. Ceiling paint is typically some sort of flat white, easily touched up without sticking out like a sore thumb. And in this case I would stay away from the wall -- just touch up the worst spots. Something good to have handy are a few artist brushes, for the occasional spot.
A couple other things...
Someone mentioned using a 3" or bigger brush. No -- a 2 1/2" angle sash brush is the best all-around thing to use for interiors.
You going to have to experiment a bit with how to load the brush. Obviously you want to scrape off the excess paint, but how hard to scrape [i.e. how much paint to leave in the brush] is something to be determined -- as you get used to cutting in you may want to carry more paint per brushload.
Someone mentioned practicing in a closet -- great idea.