Add beadboard to create interest on the non slope part of the walls
Choose one wall as your focal point (this is usually the largest straight wall) and paint with an accent color.
Wallpaper all the "straight" walls of the room, anything on a slope is painted.
Draw you eye down to the furniture by painting the lower half of the room. Again the straight walls are all painted, anything sloped or slanted acts like a ceiling and is painted that color.
The last option is to paint the walls and ceiling a neutral color and place the focus on the furniture and bedding in the room!
I hope this has helped your slanted/sloped ceiling dilemma!