According to John Patrick, Fashion designer, Organic by John Patrick.
Stick with Natural Fibers
Make sure all of your clothes are 100 percent cotton or linen, or a mix of the two. Even silk is great in hot weather as long as the item you're wearing is loose. Synthetic fibers will make you substantially hotter -- and not in a good way.
Resist the Dark Side
Create a summer wardrobe of light shades -- colors like beiges, whites, creams, and pastels. Any clothes that are darker, like a navy blazer, will just make you roast outside. Lighter shades don't just keep you cooler, but they also look more appropriate at this time of the year.
Engage in a Cover-Up
It sounds counterintuitive, but in fact you will stay cooler -- and look more put together -- if you cover yourself up. A shift dress or a light pair of pants and a top with three-quarter sleeves will look better and keep you more comfortable than a tank top and shorts because your skin will have some protection from the sun.
Tip Your Hat
Look for a finely woven straw hat, either Panamastyle or even a beachy style with a wide brim. It's not necessarily something you would wear to the office, but if you're stepping out for lunch or going to be outside, a nice straw hat can be glamorous in an old-movie-star kind of way. It's retro chic and creates a frame for your face, which makes your whole look more polished.
Slip on Heels
The ideal heel height is 7 centimeters, or about 2 3/4 inches: tall enough to be flattering but low enough to be walkable. As much as we all want to wear flip-flops, a wedge is even better because it's almost as comfortable but adds a dose of femininity. Look for a fabric version, which will feel cooler on your feet than leather.
Keep It Together
Throw on a lightweight cardigan or a short jacket with elbow-length sleeves in case you go out in the evening or the office is overly air-conditioned. It will keep the outfit looking smart without adding bulk. And go for add-ons, such as a loosely knit cotton scarf or a wooden bracelet, that don't feel hot against the skin. The last thing you want to feel is bogged down by your accessories.