What kind of sunglasses are good for flying?
No polarizing glasses, they make reading anti-glare instruments difficult and sometimes even limit your outside view if the cockpit windows are polarized as well. 
Good fit, you don't want to loose them in the slightest of turbulence. Consider fitting a strap so you can temporarily hang them around your neck. They also need to be comfortable if you wear a headset over them. 
Neutral colour (grey, grey-green, brown) of the lenses, which does not distort the natural colours too much as that would make distinguishing navigation lights more difficult. 


Actually the FAA have published a safety brochure on sunglasses which covers many aspects.