Generally, I buy less-expensive, trendy, "knock-off" sunglasses because then I can throw them in my beach bag, set them down on the concrete at the pool, etc., and it's not the end of the world if they get scratched or lost. Expensive, designer sunglasses have a definite allure to them, but 1. I don't want to drop $300 on sunglasses, and 2. I want to be able to enjoy my pool and beach time without worrying about keeping them in pristine condition.
I usually choose big sunglasses because I love that style, and they cover the delicate more of the delicate eye area. This has paid off (in addition to using eye cream religiously), because I have much fewer lines around my eyes than other women my age. I highly recommend stocking your beach bag with Shiseido's Sun Protection Eye Cream SPF 34 (my eyes are super sensitive but I can put this everywhere and there's no adverse effects such as stinging or redness).
Check out these lovely picks (my favorites are the Forever 21 Tortoise Sunglasses for only $5.80!) that will keep you safe from the sun, looking sunning, and leave money in your bank account (note the prices below each collage...I think you'll be pleased).
xoxo