The timing of your question is perfect. I'm going to be photographing the PGA Tour Championship this week with temperatures expected to be in the mid 90's and sunny skies. I've photographed this event for a number of years, and it's always hot. The golfers - especially the caddies - have to deal with the heat on a regular basis. I'm walking 18,000-20,000+ steps on a typical day. This year the forecast is for highs of 94-96 degrees several days of the tournament.
Preparing for hot weather starts with clothing. I wear a wide brim Tilley hat - the widest brim they offer and a hat that breathes. If possible, I wear light colored, light weight, synthetic clothing. Performance or athletic wear shirts are great. I wear short pants - either cargo shorts or golf shorts - made with synthetic materials, but zip pants or similar full length pants are often available. I have a pair of long pants from REI that are very light weight. Some pro golfers are wearing thin sun-shirts. I want light weight so I never wear any kind of denim or heavy cotton or leather. I choose walking/hiking shoes that are light weight or may wear golf shoes. It's important that everything I wear dries very quickly and does not hold water - so no cotton or leather.
Right behind my clothing decisions are gear choices. While two cameras is helpful, I leave my backup camera and lenses in the trunk of my car in the parking lot. If I have a problem with my primary camera, I lose an hour of shooting time, but backup gear is available. Saving weight is part of my heat management strategy. I shoot handheld. I usually carry a Z camera body externally with one lens, and two additional lenses plus a flash in a ThinkTank Turnstyle 2.0 torso bag. My typical kit is a 24-70, 70-200, and either a 14-30 or 400mm f/4.5. I may carry a 1.4 TC. I carry two extra camera batteries and an extra set of flash batteries, Zeiss lens wipes, a bulb blower, plus business cards, phone, wallet, and keys. I don't carry anything else - no cables, extra cards, or even spare change. I might carry a power bank this year - we'll see. I decide each day on whether to carry the 24-70 f/4 or f/2.8. I'm taking a lot of portraits, so it depends on my assignments. I know what I am going to need to emphasize each day, and plan gear choices accordingly. The last day I need to shoot the awards ceremony and the winner on the 18th green, so I do carry two bodies but plan a little less walking.
I make sure to drink fluids throughout the day. I'll go through 6-8 16 ounce bottles in a typical day. I try to drink two bottle of water for every one bottle of sports drink (Powerade or Gatorade). The key hear is to hydrate early and often. If you get behind on fluid intake, it's very hard to catch up. Your urine output should remain steady. If it becomes a darker color, you're getting dehydrated.
Whenever possible I try to walk and stand in the shade. Ambient temperatures are 20 degrees cooler in shade, and it makes a big difference. I'm not indifferent - I'll seek shooting locations in the shade, wait in the shade, and walk on the shady side of the fairway.
Periodically you may want to sit down in the shade and That can be a break for lunch or simply sitting in the shade and waiting for action. If you ever feel flushed, tired, or light headed, sit down in a shady location and boost fluids. Heat exhaustion and heat stroke are dangerous, and can't be ignored. I've learned the hard way to watch for symptoms and over emphasize fluids.
Conditioning is important. It takes time and a good mental attitude to handle heat. If you are complaining about the heat - even to yourself - it somehow is worse. You can't change the heat, but you can change your attitude. If you believe you are handling the heat well, it won't adversely affect you or your photography as much.
If you have a choice in planning your day, start early with sunrise and early morning photography, then take a break during the middle of the day. My normal day at the St. Augustine Alligator Farm involves a sunrise shoot nearby, early entry into the park, shoot from 8:00 until 9:30 or 10:00 AM, then go to brunch or an early lunch. Midday is reserved for image processing and possibly a short nap. I return around 4:30 PM for an afternoon session that lasts until sunset or around 7:00 PM. My routine is not too different in Death Valley, Arches/Canyonlands, or on the southern coasts. All of these areas are sunny and hot. Midday is reserved for travel and scouting.
