An elegant pair of frames is one of the sexiest accessories a person can flaunt. No one understands that better than the bespectacled Warby Parker co-founders David Gilboa and Neil Blumenthal. They’ve picked up more than a few tricks from years of watching mirror selfies go down in their stores.

How can I choose the best frames for my face?
Blumenthal: Look for contrast. If you have a round face, choose an angular frame. If you have an angular face, go rounder.

What if your face shape is a little more irregular than that?
Blumenthal: If you have an oval face, oblong frames tend to look best. With longer face shapes, you always want the glasses to fit so that you can see your eyebrows as well. Covering your brows with glasses will exaggerate the length of your face.

Is there a frame that suits all faces?
Blumenthal: When in doubt, rock aviators—they’re the most universally flattering style around.

What makes them so adaptable?
Gilboa: The magic of this silhouette is that it combines bold and subtle elements. The brow bar offers structure and a graphic line, but it’s offset by thin wire frames and slim temples. It can be sporty or dressy, masculine or feminine.

What is the biggest mistake people make with their frames?
Blumenthal: Besides wearing sunglasses at night? Rimless specs. They’re designed to hide the fact that you’re wearing glasses. But eyewear should be a vehicle for self-expression.

Have you witnessed any unexpected forms of expression?
Gilboa: When customers come in to try on cat’s-eyes, we’re privy to a lot of purring.

Should you snap a pic of your self before settling on a style?
Gilboa: A selfie will help determine whether the width of a frame is right for you. If it looks like it’s narrowing your face in the picture, then it’s too small. If the sides of the lenses extend to your ears or beyond, then it’s too wide.

What’s the best time of year to shop for frames?
Gilboa: The prime time for sales is from September to April because of the academic year. So for companies who run promotions, spring and summer are the best times to get a deal.

Warby Parker GlassesTime Inc Digital Studios

Bonus! Gilboa and Blumenthal share their top glasses picks for your face shape:

1. For a round face:
Winston actetate glasses, Warby Parker, $95; warbyparker.com.

2. For an angular face:
Watts acetate glasses, Warby Parker, $95; warbyparker.com.

For more pros’ tips, tricks, and best-kept secrets, pick up a copy of InStyle’s July issue, on sale and available for digital download now.

Want more? Check out the best sunglasses for summer in our gallery.