Let's discuss the easy stuff first, like water. I need a lot of water to keep going. I know this about myself and plan accordingly. This time of year I never leave the house with less than 3.5 liters of water. That's my minimum water allowance for hiking during the summer. I recommend carrying too much water every time. I know it's bulky and heavy, but water is the most important thing you are carrying on the hot days. When I'm on a trail that I'll be returning on and if it's a long day I will often leave a bottle of fruity sugar-water for the return trip. I stay ahead of my water needs by drinking plenty the night before and the morning of.
During peak exercise our bodies need a steady supply water to maintain efficiency. Duh. But why? When we sweat we lose volume, it's as simple as that. We excrete fluid volume directly from our bodies in an effort to evaporatively cool an already over-heating organism. What happens as you lose this fluid volume through prolonged exercise? "Well," you say, "you dehydrate.". Yes, but what that really means is that you hemo-concentrate. With less plasma volume all the electrolytes and platelets and blood cells and hormones and everything else in your blood-stream begin to concentrate. As extracellular fluid in the vascular space decreases, all those cells in the blood stream, and consequently in the muscle, aren't able to work efficiently at the exchange of sodium, potassium, and calcium ions which create muscle movement. The loss of sodium and potassium through sweating increases production and retention of lactic acid which diminishes muscle performance and can lead to muscle cramps. With advancing dehydration the body starts releasing hormones that cling to the last water in the blood stream, but this condition quickly proceeds to rapid kidney failure, cerebral edema, and eventually seizures and death. They say you can survive a few days without water but that doesn't mean you can't easily over-heat and die in a single afternoon. Start hydrated and stay hydrated.
So we lose water through sweating. Sure, but that's only one of the aspects of volume depletion. Another way we lose water is through what we in medicine call "Insensible Losses", otherwise explained as a loss that's difficult to measure and quantify. I'm speaking specifically of all the water vapor we lose when we exhale. Every breath we breath out, every expiration of C02 is combined with water vapor from our moist lung beds. Up to 25% of our water loss during high intensity cardio simply gets breathed away, and you'd never even realize it. Now that you know this about yourself let me share one small habit that can spare some of that loss. Hike with your mouth shut. Seriously, if you are hiking at a speed that requires a high heart rate, That also requires that you breath fast, so slow it down. If you're going so hard that you can't breath through your nose, slow it down. We all understand that cardio is good for us, but how does it work? Cardio workouts are high intensity and require a sustained, elevated heart rate. The heart rate goes up to accomodate the oxygen and sugar demands of your muscles (including the heart). The muscles demand oxygen as an essential ingredient in the catabolic process that creates energy from glucose. As your hypoxic drive tells you to breath faster to accomodate all these organic needs, you lose more water volume through respiration. And besides, we all know that exercising at a high heart rate increases body temperature (which is a direct by-product of the muscle "burning" oxygen and glucose [a by-product of which is lactic acid], and the increased demand for blood flow to the exercised muscle), so lower your heart rate and oxygen demand by slowing down on these hot, hot days. You're saving water, saving your muscles and organs, and operating more efficiently.
I recommend carrying some form of electrolyte replacement. Staying on top of fluids and electrolytes is always key, but especially so in the heat. Hyponatremia (low salt) and hypokalemia (low potassium) can lead to muscle cramps, renal failure, cardiac dysrhythmia, cerebral edema, seizures and death. I just wanted to point that out. If you are like me and frequently put down 20 miles in a day, you need to be taking care of yourself. I like Emergenc-C for the vitamins and -lytes. It's kinda fizzy and doesn't taste real good in luke-warm water, but you get a buzz out of the B-vitamines. I also use S-Caps! which are basically hi-end salt and potassium, by which I mean that they have an easily absorbed chemical structure. Only a small percentage of the sodium in S-Caps! is bonded with chloride (NaCl), which is a good thing. Potassium is a difficult mineral for people to absorb, but the KCl in these pills is highly refined. S-Caps were designed by a chemist who does hundred mile ultra-marathons and I find that they really maintain performance and aid recovery. I like pure caffeine as a supplement (it's been shown to improve exercise performance by as much as 20%). Be really careful with caffeine while on the trail on a hot day. Caffeine will raise your heart rate by 10-20 points all on its own, and it is a vasoconstrictor so those blood vessels on the skin that dilate to help you cool off?, they don't do as good a job and you get hotter. Plus, it's a diuretic which flogs your kidneys to excrete more urine, which of course depletes volume. Just saying. So be careful whatever you use, and if you don't have to pee, you aren't drinking enough. Understand the physiology behind what you put in your body and you'll be a better athlete.
Never ever take a break in the sun. Don't ever stop in the sun while on a long, hot day. Make the most of any opportunity to cool down. Exploit shade. Shade good.
Pay the money, get good clothes with a decent SPF rating, something that wicks, breaths, vents, shades, whatever. Get a good hat, like my infamous broad-brimmed hat that looks like it should be the official head-gear of "The Cult of the Black Madonna". It's a lifesaver.
Everybody loves visiting places like Matilija in the summer because of the swimming holes, but understand that during high summer a canyon bottom is often 10-15 degrees warmer than being on a ridge a couple hundred feet above the water. This is because canyon walls are great at retaining and reflecting heat, and due to this convection the creek bottoms get much less breeze during the day than you would higher up. There are quite a few places in the SLP that are high and piney, above 6,000 feet. Nice breezes up there. Just something to bear in mind. Well, there you go. See you out there. Adios y Vaya con Dios.