The way you part and style your hair can help you achieve the illusion of an oval face. Face shapes are generally categorized as square, round, triangular, diamond and heart. If you have a round face you can slim it down with a side part. Side parts are in style right now and are suitable for almost every hairstyle.
American Ferrera has a beautiful round face and her side part takes away from the roundness and calls attention to her eyes. Using angles and layers also helps to slim down round faces. Avoid styles that are one layer or fall just below your chin and curl under. This just calls more attention to your facial shape.
Also avoid very heavy bangs or fringe unless you are wearing your hair mid-length or in layers. The most flattering bangs for round faces are angled to one side with one side longer than the other. A bit of height at the crown also helps to lengthen a round face. The half updo is an excellent choice for a special occasion as it adds more angles to your look and helps to slim your face. Experiment with your style and ask your stylist to suggest the best look for you.