Although these are general guidelines, there is ONE rule that I always follow, no exceptions and that is:
Sometimes you have to ignore the rules and go by what you see. In other words, just like that mole on Cindy Crawford's face was the slight imperfecton that made her a Supermodel, a frame may be the "wrong" shape for you but look right on your face because of the way it tapers at a certain part of the front or because of some other quality.
     So, trust your instincts (but take along a couple of good friends who will be honest about whether a frame looks right on you): .
   Also, don't go just on the glasses that are right for your face shape but keep in mind the colors of clothing you prefer and your hair color and skin tones. The right frame can compliment all of those or stand out in glaring contrast.
       Having said that, here is a general guide of which sort of frame is recommended for different face shapes:
OVAL: If you face is basically long and your height is generally longer than the width of your face, then you are lucky, lucky, lucky when it comes to choosing eyewear because you can wear nearly any style you like.

SQUARE: If you have a very strong jawline and a generally broad forehead, ask someone you trust if they'd call your face square-shaped. Another way to figure this out is to look in the mirror and outline the shape of your face on the mirror, using a light touch, with a lipliner or lipstick. If the shape you see looks squarish, then you can assume that is your face shape (this trick works for other face shapes as well). Avoid frames that are flat along the bottom, mainly because they'll draw attention to the width of your lower and upper face and won't flatter your face shape. On the other hand, glasses with curves along the lower frame can distract from wide jaws and brows..

HEART-SHAPED:    Your face is basically heart-shaped if it has a forehead that is much wider than your narrow chin. Smaller frames are usually best for a heart-shaped face because you want a lighter looking frame that will not weight down your face and not make your face look square rather than the attractive heart-shape it already has.

ROUND: Round faces generally have similar width and heigh and some round faces look a bit chubby, even if a person is not overweight. Again, try the lipstick on the mirror trick if you have any question about your face shape. To contrast with the round shape, choose a squarer frame but BE CAREFUL. Anything that is too heavy or clunky will add weight to the face. As a general rule, frames with straighter lines are better than rounded ones for round faces.

Again, these are just GENERAL rules.  Some faces do not fall neatly into one shape and you may find that breaking the rules is the trick to finding the perfect match for your face. Have fun trying on various frames and finding the right one for your face. Eyeglasses can look attractive!