Unless you've been living under a rock, you already know that you should wear sunscreen when you go outdoors; most of us also know that we should do so even on cloudy days. But did you know that your eyeballs can also get sunburned? This is why, in addition to looking great -- and much more importantly -- the sunglasses you choose should be ones that shade and protect your eyes. They're not just a cute accessory anymore.

Rules for Protection

The first thing you want to consider before you ever take fashion in to account, is how well your shades actually protect your eyes. Aside from making time out in the sun more comfortable (we've all gotten a headache from squinting at least once, and it's not fun), your sunglasses should offer protection from both types of ultra-violet rays: UV-A (blocking at least 90%) and UV-B (blocking 99%). Most pairs have a sticker on them that indicates that they offer both kinds of protection, but you can also find this information on the tag. When purchasing a higher-end pair, ask the salesperson -- they should know everything there is to know about the item you're about to shell out for. And here's something else you may not know: darker is not better.

Another rule-of-thumb to keep in mind: in this case, size does matter. The bigger the shades, the more of that delicate skin around the eyes they'll shield, so go ahead and channel your inner Jackie O. -- but you still want to apply sunscreen. You can't ever be too careful when it comes to sun damage prevention, especially for the eyes.

Guidelines for Style

After you narrow the selection down to include only those shades that offer adequate protection from UV rays, you can indulge in the fun part: choosing the pair that makes you look fabulous. Check out this great slideshow from "the Bible" (A.K.A. Cosmopolitan) that outlines which styles look best on certain face shapes and includes pics of the celebs that boast each shape. It also describes the predominate features that determine your face shape and suggests two (affordable!) examples for each. The gist of it is this:

Heart-shaped faces are best-accentuated by cat-eye frames.

Oval-shaped faces are flattered by any style, so go wild if you're lucky enough to be an oval.

An angular (square) frame is best to offset a round face.

So naturally, a round pair will look best on a square-shaped face.