Finding a great pair of sunglasses is a must!  Personally, I suggest having at least two different pairs.  But if you can, have a whole sunglass wardrobe based on your mood, outfit, or location.  You’ll need a casual pair, dressy pair, very dark pair for the beach/pool/water, and a lighter pair for those partly cloudy days.  You can then add in a few differently styles depending on how fashionable you want to be.  The large Jackie O variety, retro 80s styles, or classic aviators are still considered fashionable.  Here are a few tips for finding frames that suit a square shaped face.  You can employ these same tips when looking for regular eye glasses as well. 

The Square Face
A strong jawline, a broad forehead, and a wide chin and cheekbones characterize a square face.  Width and length have the same proportions or are equal.  Frames should make the face look longer.   Gently curved narrow styles will minimize squareness and add length.   Frames should be more horizontal than vertical.   Try oval shapes with temples in the center.