Last week I helped a client select eyeglasses and RX sunglasses (from the more than 3300 Schaeffer EyeWorks has available!) To some, frames this may seem like just another accessory, but keep in mind that this "accessory" is worn directly on your face and helps create the image that is the first impression you give someone, especially if they make eye contact upon meeting, as they should. (See the pop-culture inspired sunglasses chart at the end of this post as evidence of just how important what shades one chooses can be.)
As far as which frame for particular face shapes, all you really need to remember is "opposites attract." In other words, if you have a round face, avoid round frames and try square instead. If you have a square face, try oval frames. Hair and skin color also comes into play. You want to coordinate, accentuating undertones or highlights, but be careful not to match, which can wash you out.
When it comes to sunglasses, you can allow your aesthetic preferences dictate a bit more. My client, like me, likes a dark lens with a lot of surface area (covering above and below the eye, offering the added benefit of wrinkle prevention) and some peripheral coverage. So a Jackie O style frame with ever-so-slight wraparound fits this bill.
If you have a classic style, consider wayfarers, which are versatile in that they can be dressed up or dressed down. On that note...guys, sporty frames like Revos or Maui Jims - especially with with cables attached - is not appropriate for suited or dressy casual looks.
No matter what you choose, protect your peepers! They're the only ones you'll ever have!