I work outside. Here's what I've learned. (That I haven't seen mentioned.)
Taking aspirin before you get hot helps keep you from getting a headache. Citrus works too. I like lime-aid with minimal sugar.
More than one quart of Gator-aid is not good on the kidneys. I'll make a quart last all day by drinking water too. (Not iced cold and don't chug.) If you aren't peeing, you're not drinking enough.
Body powder for your feet and crotch. I buy Gold bond Ultimate but some of my co-workers like a brand called Fresh Balls. (When I work I wear wool socks and boots. When I golf it's cotton socks and tennies.)
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This is real advice here on the Gatorade. Everyone needs to stop and think about how much they are drinking of this stuff. I worked a few triathlons in the medical tent and 8 out of 10 people would come in with electrolyte imbalances because they were primarily drinking Gatorade. Water is your best friend and I for one find that super-cold water can sometimes have a negative effect on your body in the extreme heat. If you can learn to drink temperate/warm water your body will metabolize it better. The more you drink the better you will feel. Stay away from caffeine and alcohol and cigarettes, all these things will take everything out of you. I don't drink beer when I'm on the course but I will smoke and every time I know it's gonna drag me down and it does.
Also, I can't stress this enough, don't pour cold water on the back of your head near your neck to cool down. This will produce a negative effect on your body and can ultimately make you feel much worse in the long run. I learned this after years of working outside in this kinda heat from some old-time laborers and it seemed counter-intuitive but it really matters. The bodies thermo-regulatory system is located at the back of the brain and by pouring cold water on that portion of your head in this heat it does produces an instant feeling of refreshment but it isn't good for you to keep doing that unless you start feeling like you have heat exhaustion or heat stroke and then I would say go for it by all means. If your not super-heated and delirious avoid doing it.
It's always good to wear wide rim hats, I normally wear a Bonnie Hat with a Chill-It's bandanna. Chill-It's are a great little item that cost's about 5 bucks, soak it in cold water and it's good for over 12 hours and it will keep your forehead nice and cool unlike conventional bandannas.
Wear dri-fits if you got them and always go for light colored clothing. I sweat big time and I often will have a few t-shirts with me if I'm gonna play multiple rounds so I can change them. If your playing in certain areas you may need to wear pants, I normally wear jeans to avoid poison ivy/oak/sumac in wooded courses and boots to avoid snakes after I had a run in with a rattle snake years ago.
Eat well before you intend to go out in this weather, not eating enough and having low blood sugar will take away all your energy. Bring some fruit with you like an apple or granola, they can keep well in this heat and are convenient.
One thing that I do if I have been inside all day and I intend to go and play, on my way to the course I will not run the AC at all so I can start getting used to the heat.
Also warming up before you play is a good idea so you don't cramp up right away.
Just my advice and adding to what others have already said.