Should you have a heart/triangular face, stay away from frames that are wider than your temples, those with an exaggerated upward and outward sweep, or those which are top and side heavy.

All these frames emphasise a narrow chin. Select frames that have the emphasis on the lower outer corners.

For round faces, always avoid perfectly round glasses.
They can make your face even more round. All these frames emphasise a narrow chin. Select frames that have the emphasis on the lower outer corners.

Most frames will suit an oval shaped face.
Use the natural balance of your face shape to take advantage of a broad range of frames in proportion to your features and face size.

For square faces, frames that are too square or angular will exaggerate the squareness of the face further.
"Droopy" shapes will also bring attention to the lower face, accentuating the jawline may be what you are looking for.

For those who want to soften a square face, look for frames that are curved at the bottom corner and side.