I'm a firm believer that a pair of sunglasses can completely transform your entire look. Sometimes that's a bad thing, as was the case with the massive white shades I sported all throughout college. Since then, I've learned from my mistakes and now adhere to a few simple rules to ensure that I find a pair of glasses that fits just right.
1. Know your face shape (and the frames that are most flattering). These shades were literally the first pair that I tried on in the store since I knew that the updated Wayfarer's size and slight angle would work for me. Anything overly circular or too small is not my friend. Neither are yellow tinted frames (another thing I learned while browsing through old pictures from high school).
2. Be aware of what's current, but don't splurge on anything too trendy. I'm all about the cat-eye trend, but I'm also am not going to dish out an exorbitant amount of money on something that will be outplayed in several months.
3. Pay attention to colors. This includes your hair, skin tone, eye color and the clothes you tend to wear. Since I'm blond with light eyes, I only buy black glasses as I find it makes the most dramatic statement.