Paint the entire walls and ceiling a light, airy color to give the entire room, a full and expansive feel to it. Colors like pastel yellow and cream give a more open feel to the room. You'll have to paint the walls and ceiling the same color as there's no spot where the walls end and the ceiling begins.
Install floor length curtains to all the windows in the room, using fabric that has skinny vertical stripes. Just as vertical stripes can make a short person look taller, they can have an elongating and lengthening effect on a room.
Fill the room with high-backed furniture that has a strong vertical component. For example, if your curved ceiling is in a bedroom, add a bed with four skinny posters. If it's in a dining room, add narrow, high-backed chairs.
Install recessed light or track lighting that directs light upwards to the ceiling, causing it to have greater illumination, and a sense of dissolving upwards.