For starters, hydration is often key to feeling not as hot. As photographers, it's important that we do not forget sunscreen and drink plenty of water. Comfort is key and, while it's almost impossible remaining comfortable when the temps climb up above the 100 degree mark, you want to make sure you take steps to make yourself as comfortable as possible. Lots and lots of water, sunscreen, and a hat but be careful to make sure your hat does not cut off or vignette your camera view. Don't get your hat in front of your lens and you'll be fine.
Shade is your friend as a photographer in the heat. Even if you are standing in full sun and shooting into the shade, make sure you spend some time ducking into the shade for some cover from time to time, just to keep yourself from overheating.
Timing is everything in photography and here too it can help you. Here in Texas, the hottest part of the day hits us about 4 or 5 o'clock. This is also not the greatest time for good light conditions. If you can, to beat the heat, I recommend shooting in the golden light as much as possible. Think early morning and just before sunset. These times not only give you great light but they can offer up a break from the heat. As the sun gets low in the sky it makes for better photography and a cooler photographer so make the most of it. The key is find out when the hottest part of the day is and try to avoid working during those hours, instead concentrating on times where it's a bit cooler.
Don't rule out night shooting. Summertime is a great time to get outdoors at night, after that brutal sun has set in the sky. Don't be afraid to try your hand at some long exposure shots after dark or even to mull around some city scenes to shoot things like shops after dark. This is a great time to do it.
Finally, think about lighter gear. Summertime can be a great time to get out the iPhone and work some mobile photography. Packing a lighter camera can help you feel a lot less of the heat stress, since you won't have that big gear to lug around. It can also free you up to shoot things like food, indoor shots, and the like, which always work when it's hot outside. Think about places where people go to cool off, such as watering holes, swimming pools, the beach, or an indoor paradise like the mall or the movies. If you've always wanted to shoot some of these places, why now is a great time. Work with what you've got and make the most of it, for winter will be here soon enough.
I hope these tips come in handy for these dog days of summer, and I hope you keep shooting, even in this blasted heat.
Until next time...
PS This one taken on the ferry up in Washington State. Another way to get out of the heat is to plant yourself on a boat, even a ferry like this one will do.