To soften too sharp features, go for earring designs with curves or rounded. Hoop earrings work very well as would earrings with round beads or with circles in the design, or even waves.
To Sharpen flat features, wear triangular, square, or star shaped earrings. Shapes that are pointed or have sharp lines work well.
For a round or square face, you want the illusion of length. A dangling pendant or a cascade of beads that draw the attention along vertical lines would visually slim and lengthen your face. Shoulder dusters are your best friend, as are drop earrings and chandelier earrings.