The heat does not bother me much, I'm more of a whining cyclist in the Winter... but if it's really crazy hot and humid I bring extra water and electrolytes, and sprinkle some cold water over the skullcap and the jersey every 5 miles or so while I'm riding.
Some friends that ride with me do not tolerate the heat well, and bring little ziplock bags with crushed ice. They just put three or four of those baggies in the jersey pockets or in the cage rocket holder, and get some ice out when they need. I've never tried it but it works for them.