The fact is that big hybrid meat birds, like Cornish Cross, really struggle in the heat. They are so big and have such fast metabolisms. The best tip I can give, is to try to manage your schedule so that you are not raising them to maturity in the peak of summer. I like to start mine in the Fall, so they are maturing in November and December.
If that is not realistic, I would suggest a few things. First, if possible, pick a slightly slower growing meat bird, like a Red Ranger/Freedom Ranger/Slow White Broiler. Second, make sure they have areas of deep shade to hang out during the day and very easy access to water. If you are dealing with a dry heat, misters really help. Finally, make sure that wherever they are sleeping, is spacious enough so they are not being crowded and very well ventilated. You will have to a plan to deal with their droppings on a daily bases, to keep the smell and flies under control.