Some of you guys probably know this already, but I thought it would help a little, with your hot weather riding. I picked up some polyester breathable sleeveless t shirts from Target, that I wear under my jersey and it keeps me cooler than just wearing a jersey. It gets soaked from sweat, but so does my jersey, but it keeps me cooler. I do most of my riding around Houston and it's pretty hot. I was watching the landscapers for a while and I went over to talk to them and asked if it helped wearing long sleeve shirts and rags around there heads and they all said yes.
That's when I thought I would give it a try and it does work. I just went back and bought 3 more. They're only $10, if you would like to try one. Good luck.