The round face shape is a soft shape with often grants the person a youthful look (win-win!). In this case you often wish to add a little sharpness and this can be done by choosing glasses with a square or rectangular shape. It is often also a good idea to choose the more chunky versions of a shape to add character to your face.
Again - remember that a circle might be a strictly defined shape, but a round face shape will never be a perfect circle. You just have to find the shape that resembles your face shape mostly.