Most of my photography is conducted outdoors. That said, since I live in south Florida there is a lot of sunshine, heat, and humidity. Regardless of the weather I always wear long pants and shirt, a floppy hat, and a light sneaker. Although it would be cooler with shorts and a short sleeve shirt, I want protection against UV, bugs, and I never know when I might be on my knees or belly. Just last weekend I came across a gopher tortoise and was able to lay on the ground inches away from the tortoise for some nice portraits. Once completed, I got up, brushed off my shirt and pants, and was on my way. Nothing is worse than trying to brush off sand from wet sweaty knees, arms, and elbows, etc. This is also a great way to get sand on your camera equipment as the sand dries and falls off as you continue with your photography.
Due to the heat and humidity I always have a bottle of water on me at all time. The bottle of water is on my check list so I never forget to bring it out in the field. Recently, I added a new item to my check list as it relates to staying cool. This past Father's Day my daughter gave me an Endura Cool bandana (check out missionathletecare.com). You get the bandana wet, snap it to activate it, and then I wrap it around my neck. Is it better than a cotton bandana I don't know, but I have found that I do cool down faster at the end of my outing. No, it isn't as if someone dropped an AC unit down your pants and that you'll no longer perspire, but it does provide me with some cooling effects when I'm out in the field. You might be interested in checking these out. Stay cool and hydrated.