Summarize:

Picking the most ideal frame for your face is not entirely up to you. Much is dictated by the natural features of your face. The most important aspect of this is your face shape. You can easily figure this out by looking in the mirror or taking a picture, and comparing it to a diagram.  Round face. With this shape, look into more toward square and rectangular frames that will make your face look slimmer and longer. Avoid frameless, oval, and circular frames. Oval. Pick frames with a strong bridge, and avoid large frames that will make your face look small. Square. To offset the angularity of your face, tend toward rounded or round frames. Diamond. You may not want to accentuate your narrow forehead, so don’t pick wide frames that call attention to this. Opt instead for small, rounded frames. Heart. To minimize how large your forehead appears compared to your chin, pick frames that sit low on the nose. This makes the center of your face seem lower. If this is not your first pair of frames, you probably have an idea of your skin allergies. Otherwise, your dermatologist can give you a test to determine this. If you are unsure and don’t want a test, there are materials that are more likely to break out your skin than others.  Plastic or synthetic. These frames are often designed to be hypoallergenic, meaning they are less likely to break out your skin. They also have a wide price range. A few examples are cellulose acetate/zylonite, cellulose propionate, and nylon. Metal. Metal frames vary, as far as skin allergies are concerned – some are hypo-allergenic, but others may break you out. Examples are titanium, stainless steel, beryllium, and aluminum. Other/natural materials. Wood, bone, and horn do not usually cause skin allergies. Most people fit into two basic categories for skin tone. To figure out if you have warm or cool skin tone, hold up a piece of white paper next to your face. If your skin seems yellowish, brownish, or bronze, you have a warm skin tone. If your skin seems pinkish or bluish, you have cool skin tone.  For warmer skin tones, stick with tortoise, browns, and dark green over white, black or pastel colors that contrast heavily. For cooler skin tones, look for more stark colors like black, white, and bright colors. The more brownish hues will contrast with your skin tone. This is similar to your skin – there are two main categories of hair tones. Some cool hair colors are strawberry-blonde, blue-black, and white. Examples of warmer hair colors are brownish-black, golden blonde, and gray. Apply the same rules for frame color as with your skin tone. If your hair color and glasses frames don't look good together in the optical, then you will hate the glasses at home!
Determine your face shape. Figure out skin allergies. Look at your skin tone. Consider your hair color.