Start with a good, wicking skin layer. There's plenty of choices out there. I've been using Cycle Gear's house brand Heat Out stuff. It's cheap. gives you some fatigue-delaying muscle compression, and really helps you manage the sweat. I've also been using SiDi's silver-content wicking socks which are comfy and prevent the "swamp foot" you get with cotton socks in hot weather. Combine those with a pair of ventilated Riding shoes (SiDi Doha's are my current favorite), and your feet are good to go. Armoured riding jeans work well with the skin layer, and ventilated textile riding pants are better yet. I wear a ventilated motocross jersey over the skin layer (again, Cycle Gear's BILT line is dirt cheap and works great). While any ventilated MESH armoured riding jacket will do, I've had good results with both my Cortech DS Air 1/2's and Olympia Airglide 2/3's. I also wear a helmet liner (Heat Out, Head Slicks, etc), which helps to keep you cool as well. Add one of those cooling vests, cooling neck wraps, or simply soak your shirt/skin layer with cool water in a pinch, and you're chillin' like a villian.
Drinking plenty of fluids often (good time for a riding break) is a must, as well as making sure you're eating small nutrient-dense/fat calorie sparse meals every two/three hours is always a good idea. It keeps your blood sugar and energy levels constant to delay muscle and brain fatigue.
While no helmet, shades, a tee and jeans may make you look cool, you last a lot longer in the heat if your control you exposure to the elements and manage your perspiration. In addition, the aformentioned "cool" riding apparel won't do much for you in the event of misfortune.
Regards,
Kevin M Hogan
MSF #121548
NJ MVC DCN 618