I'm not an expert on this. I've done a little research and this is what I've come up with (plus some personal experiences).
First off, I have a very square shaped face. Even with hair styles, I find that soft curls look better than pin straight hair. Why? Well, the straight lines tend to emphasize the square shape, which isn't what I'm going for.
Theoretically, this works with sunglasses too. I look better in rounder (circular) frames than square ones. Sometimes I can get away with square frames if they're softer at the edges.
My sister has a rounder/oval-er face. For her, square frames look better because it gives her face a little more structure.
For heart shaped faces, soft square frames and aviators will work.
For round shapes, rectangular/square frames work well.
Of course, it varies from person to person, since we all have different facial features as well. So go out there and experiment!
Let's get ready for summer~
Until next time,