I am a fan of my frozen water bottles (I use the 1 liter bottles). I fill them about 2/3-3/4 of the way up and store several of them in the freezer for the next day, topping off the empty space with my delicious tap water (not everyone has access to quality tap). I drink it as it melts (it's in one of those bottle wraps), and refill it as I go, if necessary (and it usually is!). I keep 3 bottles in rotation, year round. The envious looks I get are priceless!! The sweating bottles definitely have a psychological effect, too. Luckily, I am not an excessive sweater.
We used to wear polyester uniforms. I found that wearing a white cotton undershirt under it did wonders for my comfort level. Lay in a supply of cotton undershirts. I promise you it will not make you hotter. I often wondered why firefighters wore them; I get it now.
In Jazzercise, we used to freeze wet washcloths (keeping them in a little cooler, in little indiviual plastic bags) and pass them out after the aerobic portion of the class. We only needed to do that in the summer, and they were godsends, making the rest of the class tolerable.
I am super-anti heat and humidity!!!