Why is it that some hairstyles look great on your sister, but less than great on you? Just like other types of fashion and accessories, some hair looks best on certain body types (or in this case facial types). Your body is completely different from anyone else's and thus how you wear your hair should be personalized to your face shape. How do you know then what hairstyle will look best for your face? (If you're unsure about what your face shape is check out What is My Face Shape?) Once you have the basics down, it will be much easier to determine what flatters you the best.
For oval faces, really any kind of hairstyle will look great. (An oval face is a great shape because it's so versatile with any kind of facial accessory—you can do anything you want!) You look amazing with short hair, long hair, and anything in between. The best way to flatter yourself is to pull your hair back away from your face (with accessories like barrettes, bobby pins, clips, etc.). The only things to avoid are heavy bangs (with this facial type you want to accent your shape and heavy bangs completely cover up your forehead)
If you have a rectangular face, medium to short hair looks best because it balances out your face better. Soft bang look good with this facial shape. A shorter cut will also shorten the length of your face a bit while long hair will elongate your already longer face. Avoid that! Hair with layers is also good for balancing because it adds volume to your hair.
If your face is round you want a shorter style that's really full and adds a lot of volume to your look. Wear side parts and avoid styles that accentuate and add to the roundness of your face; you could end up looking chubby when you're not. Steer clear from super short cuts and straight bangs—bad idea. For a square face, you want a short to medium style that is more rounded and soft at the tips so it softens the line of your jaw. Don't try styles that are really long and draw attention to your sharp jawbone. For the heart shaped face person, do styles that are no shorter than chin length. Swept up layers around the face add emphasis where its needed and side parts work best with this face shape. You don't want short styles that add height to the top of your head.
Here's a recap:
Now that you know what hairstyles look best with your face shape, you should have no troubles with that next hair appointment!