I recently started using a neck fan (sorry no URL, google neck fans). I did NOT get the one with the open fan blades, the one I got has the fan blades enclosed.
For a while I did not think the neck fan was helping much, but on Sunday I forgot my neck fan when I rode. In spite of my ice vest, technical fabric riding clothes (including socks), and ventilated half-chaps I got heat sick (I have MS, heat sick means I lose 1/3 to 1/2 of my physical and mental abilities.)
Another thing I use is Honeysuckle flower tincture (Lonicera japonica). I started using too much (dropper full) and it affected my bowel movements weirdly. I did not use it on Sunday and my mental abilities were SO MUCH WORSE that I decided I would go back to it, but now I just do three drops in a bottle of water, and I am hoping I can build it up slowly. It did help me today when I had my lesson with just three drops in my water.
When I started using the Honeysuckle flower tincture years ago I noticed that it seemed to thin my sweat so it evaporated more easily and quicker. It is not in the normal literature, I learned about it searching through a Traditional Chinese Medicine Materia Medica. I read the description, realized that it may well help me when I got sick from the heat, and it really helps me stand the heat. Just start off slow to let your system adapt to it.
I had my lesson today. I was hot, I was miserable, I could not do anything vigorous at all (just walking and 2-point at the walk), but I did not get heat sick, just hot and sweaty. The neck fan and Honeysuckle tincture help with the brutal Southern summer heat and humidity along with the technical fabric summer clothes. I was so glad not to get heat sick again!