last summer I was working for another company (i thought I was going to be selling mine. Kept mine running, and then decided to keep it going.) Anyways our attire was brown duck carhartt pants, and a green coton polo or t-shirt. Plus a hard hat and saftey vest (large commercial site). You just need to drink a lot of water, but also get the electrolytes in you too (gatorade). Lightweight long sleeves help too since it absorbs your sweat and then keeps you cool. Wear a hat, and get a wet towle or bandana and put in on your head over the back of your neck.
We do the same thing with water. Fill up gallon jugs the night before, and freeze them. Then take them out and they thaw out, and if you need too, add a little water to them, but those things can last sometimes almost 2 days. And I also bring about 2 bottles of gatorade to drink at throughout the day. No soda at all, that will just dehydrate you. We do those types of water bottles everynight, but they work wonders in the summer. If your really thirsty, and the ice hasnt melted a ton, at a stop, put it on the ground by a tire, it will melt really fast, but still be cool.