Inspect your room for current trim work. Survey the floor, door casings and chair rails for the woodwork style. The current trim design may help you select new molding. For example, if your door casings are rather simple an ornate crown molding might look out of place.
Determine the size of your room. Larger rooms look better with thicker molding and smaller rooms look better with delicate molding. "This Old House" magazine suggests that crown molding for 8-foot-tall ceilings should range between 2 1/2 and 6 inches tall; molding for 9-foot-high ceilings should range between 3 to 7 1/2 inches; and molding for 10-foot-ceilings, 3 1/2 to 8 inches tall.
Combine several pieces of molding together, or "build up," to get the design you're looking for.
Add molding to the wall, and then attach some to the ceiling for a more detailed look. Adding detail to the ceiling gives the illusion of height in a smaller room.