I kind of enjoy riding in the hot Texas summer so I work up to it gradually, starting in springtime to improve heat adaptation. I stick to moderate or easy efforts for 30-60 minutes, two or three times a week -- what some training guides call Zone 1 or 2 at most. I do my harder training efforts indoors or in the early morning or at night.
I just got a Wahoo Tickr heart rate monitor to be sure I keep my efforts in the low to moderate zone for hotter rides. At 61 I know I won't be able to tolerate the midday heat as well as I used to, and HR is a fairly good indicator -- although it depends on meds and other factors. The one OTC med I take most often that affects my heart rate and blood pressure is Sudafed for sinus congestion.
Alcohol, even a single beer, can dramatically affect my heat tolerance. I learned that years ago when I was in my 20s on some 50-125 summer rides in Mexico. I've cut way back on the beer, usually drinking only 2 or 3 a month now with food.
There's a theory that, besides body fat, excessive sugar consumption can adversely affect heat tolerance and even sunburn tolerance. I'd already cut way back on the sugar too and am within 5 lbs of my minimum body fat weight, the least I've weighed since I was in my teens and early 20s. I've been riding without sunscreen this spring. So far, so good, no sun burns. Not very scientific, but I know I used to burn much more easily when I was overweight and ate lots of sugary stuff.
I'm also curious to see whether there's any measurable benefit to Vitamin D and bone health, and natural production of nitric oxide, from moderate exposure to sunlight. I'll probably carry a small container of sunscreen or wipes in case I'm out longer than an hour.
And those theories may be shot down soon by counter-research, so I'm still fairly skeptical.
Besides carrying and drinking plenty of water, I use the Polar insulated bottle to freeze the water/electrolyte solution ahead of time. I fill it half full, freeze it, then add water to top it off. It'll usually stay cool for hours that way. On the hottest days I'll carry a stainless steel double insulated thermos with frozen electrolyte solution. That'll stay frozen for hours. I've had some 12 hour summer days on the bike (plenty of rest breaks along the way), and the contents were still cold after dark. I have to carry a non-insulated or lightly insulated bottle too, because the stainless thermos won't thaw for hours.