Take and record the measurements of the floor, walls and ceiling surfaces. Make a diagram or map of your room. You will need measurements to buy nearly everything for this room. While paint can be estimated, a good record of dimensions is critical for wallpapering and assessing the scale of furniture.
Paint both walls and ceiling the same color to make a room seem larger. Pale colors will both make the room feel more spacious and enhance available light. Angles will become less noticeable if they blend into wall space. If you are using wallpaper or contrasting paint, divide the space vertically rather than horizontally so that the eye is carried upward. The standard white ceiling and papered wall horizontal division of space will make the ceiling seem heavier and more oppressive.
Arrange lighting to emphasize ceiling height. Positioning track lights at the top of an angled wall or hanging thin, teardrop-style pendant lighting will make the ceiling seem higher than heavier, wider chandelier-style fixtures or nothing but floor and table lamps. Some ceilings may accommodate recessed lighting.
Keep window treatments minimal to keep as much natural light in the room as possible. Fold-back window shutters permit daylight and provide privacy when needed. Left natural or painted the color of walls and ceiling, they make small windows seem larger. Use sheer curtains for the same reasons, keeping them close in color and tone to the paint. For a touch of color, consider a looped sheer scarf valance or a short fabric valance hung above the window frame to extend the window size visually. If space permits, enlarge windows visually by edging them with a wallpaper border.
Install a skylight for maximum visual expansion of your slanted ceiling. If a skylight is beyond your budget, use reflective accessories to make the most of existing light. Place mirrors, polished metal and high-gloss ceramics where they will reflect window or lamp light.