Paint the slanted ceiling walls an accent color that subtly contrasts the colors of the rest of the room. For example, if the room is cream, paint the slanted ceiling yellow. On the other hand, paint the walls that frame the slanted ceiling an accent color. Thus, if your slanted ceiling is cream, paint the framing walls yellow.
Install distinct crown molding all along the top walls of the room. Paint it white or even gold to help distinguish it from the sloped ceiling. This will give the slanted lines more emphasis.
Hang an appropriately sized picture or print at the base of the sloped ceiling. This will seem like a completely counterintuitive place for a piece of art, but it is a striking way to showcase the wall. Alternatively, hang a series of pictures along the base of the sloped wall.
Hang Roman blinds or balloon shades along the windows of the room. The fabric of the window treatments will fluidly mimic the lines of the sloped ceiling, creating an overall harmonious environment.
Select chairs for the room that have angled backs that mimic the angle of the sloped ceilings, in your chosen decor style. For example, if your style is modern, the sloped back of a Herman Miller chair would be compatible with the sloped ceiling and help accentuate it.
Space furniture away from the sloped ceiling, but around it. Furniture should loosely surround the sloped ceiling like chairs in a theater surround a stage. This will help emphasize the sloped ceiling.