I got through a lot of these, as I tend to forget them in airports, but the gist of my strategy is this:

• As dark as possible
• I prefer non-polarized, as polarized sunglasses tend to interfere with plane cockpits and car HUDs
• Big enough to cover most of my sight angles
• I never liked those thick nose bridges, as I find them too hipster-y
• Other than the above, IDGAF about how they look. I wear them for me, not for others.