JulieFlyGal, I'm sure this has all been said before.
Stay away from polarised lenses. I've flown with them and found it very hard to see instruments; not glass cockpit either. Mirror lenses are a no-no I'm told, but I forget why. I chose a grade 3, brown-tinted lense because all the greys I tried seemed too dark. Some say brown lenses are rubbish for flying but they work for me.
Choose a pair with thin arms. Better fit under headset earseals (smaller gaps), therefore better noise reduction.
Go to a store to try the sunnies on before you buy them! No good buying what looks good in a photo, only to discover later that you look like a complete pillock with 'em on.
But most important of all, spend as much as you can possibly afford on a great pair of lenses. Your eyes take a hammering up there. Shell servo sunnies definitely not recommended.
For what it's worth I wear Ray Ban (the poor man's Serengeti's or Maui Jim's) Sidestreets RB3183 and I'd recommend them.
Cheers.