Now, I've covered the shoe and bag thing many times, on this blog and elsewhere. When it comes to sunglasses, I personally prefer black or tortoiseshell frames in a classic shape. Nothing trendy (colored lenses), blingy (tons of rhinestones or logos), or just weird (oddly shaped lenses, etc.). I also think that most people should err on the larger side when it comes to sunglasses - besides offering better protection from the sun and the elements, they just look more glamorous.
Some of my favorites:From left to right: Guccis from Neiman Marcus, Marc Jacobs and Versace, both from Eluxury.
You don't need to drop megabucks on your shades, however. You can find similar silhouettes at places like Target and Forever 21 (obviously, these won't be as high quality as their pricier alternatives, but if you have to go cheap on something, sunglasses are probably the best option. Just stick with solid black).
Target, and the pair on the right is from Forever 21. Cheap and chic at its finest.