The part about not letting wall color up on to the ceiling is
important, I find. Better to let the white from the ceiling white go
just slightly down onto the walls.

Just from experimenting, I find that the same rule is important for
trim around windows, trim along the floor, etc. It has to do with which
color is darker, how the natural or artificial light will hit the area,
etc., I suppose. But in any case, in most of my rooms, letting the wall
paint go slightly onto the trim always looks much better than the
reverse. In other cases, it needs to be the reverse or it looks like
crap.