Sometimes it's so hot, the only accessory you can really rock comfortably is a pair of great sunglasses. (Don't shades always make you feel cooler? Somehow, when my eyes are protected from the sun, it seems less steamy.)
And a good pair of sunglasses is an investment worth making; the quality of the lens will be higher and protect your eyes (always check to see that UVA and UVB rays are blocked) and they will be less likely to break, crack or leave you with a pair that's taped across the bridge of the nose because they've snapped in half.
Just don't sit on them (!) and you'll have just about the most perfect sunglasses ever.