It's true, your glasses can either enhance or detract from your overall look. This is why it is important to assess if the frame suits your face shape besides gauging other factors like comfort and functionality. Here's a look at the choice in frames for different face shapes.
Round
Round faces are as long as they are wide, often with full cheeks. If you have a round face, opt for angular frames to add more dimension and definition. To create an effect of length, you can choose between full rim and semi rim frames. To add more sharpness, go for rectangular (Hilary Clinton's rectangle glasses make her soft, round face appear more chiseled) or square shaped frames. Wayfarers also suit this face shape well (think Austin Power's square wayfarers).
Oval
Typically three parts long to two parts wide, oval is a versatile face shape that can lend itself well to just about any glass frame. Only narrow frames and heavily embellished ones tend to add undesirable length to oval faces. These apart, various frame silhouettes, from rectangular, square and horn-shaped to oval, round and wayfarer will easily suit your oval face. Some oval-faced celebrities include Meryl Streep, Dan Aykroyd and Woody Allen, who wear wide rectangular glasses, square shades and wayfarers respectively.
Square
Though a square face does not drop narrowly from the forehead to the cheekbone or from the cheekbone to the jawline, it is widest along the jawline and forehead. As jaws are prominent, it is best to avoid glasses that add length to the face or sit high up on the nose. Instead, choose less angular frames like ovals, rounds and wayfarers that complement your defined features. Semi rims and rimless frames suit this face shape very well. Examples include Johnny Depp's round wayfarers and Tina Fey's oval frames.
Heart
A heart shape is wide at the forehead and narrow towards the jawline. The idea is to keep the focus on the top of the face. This is possible by avoiding oversized frames, highly embellished frames and bottom-heavy frames that bring attention downwards. To add a softening effect, consider rimless frames in the style of Ryan Reynolds. Other options include ovals, rectangular frames and wayfarers (think Jennifer Aniston's delicate wayfarers).
Triangle
In faces that resemble a base up triangle, the top third is wider than the bottom third of the face. The cheeks are full and chin is prominent. Angular and edgy frames can add a lengthening effect to this face shape, while prominent or strikingly designed frames can soften the effect of full cheeks. Wayfarers (think Hillary Duff's oversized square wayfarers and Philip Seymour Hoffman's rectangular wayfarers) and horn shaped frames suit this face shape.
For a visual check, google the celebrities we've mentioned in the article and make an informed decision.