Read this, it helps:
http://www.kayaklakemead.com/dehydration-heat-exhaustion-stroke-prevention.html
Also, they say your water bladders loose water if something is pressed against them, but I got one of these to prevent that. It's cheap and easy to install:
http://www.rei.com/product/727447?vcat=REI_SEARCH
Some other tips, are to wear white like posted above. I always wear a white t-shirt for this very reason. Also, wear a hat of some sort to keep the sun off your face and shoulders. Wear baggy clothing too, tight clothing allows more contact between your skin and the heat. Baggy clothing creates a "shaded" barrier between you and the sun. I always take some Tylenol in case I get a headache. Also, stay away from rocks and such, they hold in the heat for hours and hours and hours, sometimes overnight if it's hot enough during the day. If your hiking in a super hot region, like around central AZ or Death Valley, it probably won't get cool at night, so plan for that. You'll sweat at night also. During the day if you need to rest, don't sit on the ground, find a rock in the shade, that's been shaded all day like under a tree, and sit there. If you sit on a rock you're going to get hotter because there is more heat near the ground than where you head is when your standing up. It won't do you any good to sit, it'll make you worse. Find shade, either from a cactus, or a tree, or a cave. If you get really really bad, and you have a small shovel, you can sometimes dig into the dirt a little and find some cool dirt. Only do this if you have the energy though, as exhausting yourself digging could be detrimental.
Some other advice is to find your water sources BEFORE you head out, if your hiking. It's best to know where you need to be, than to get over heated and have to hunt for water. Also, if you get frustrated, please stop and rest in the shade. Irritability is a very early sign of being over heated. If you keep pushing it you'll end up with a stroke.
NEVER GO ALONE. Ever. I don't care what your doing, or where your going, but always take someone with you. For one you can do twice the work if need be, and for 2, one of you will have a stronger tolerance to heat than the other, and lives can be saved with a buddy. Another tip is to always let someone know exactly where you'll be and when you'll be back. Then, if your gone too long, they can send someone looking for you. Try not to stray too much off the planned path, it makes it easier for rescue crews to find you.
Also, please please please go prepared. Take emergency supplies. It's better to be safe than sorry. And don't forget matches. Just because you're in the desert don't mean you won't need a campfire at night. They're not just for warmth, but to fend off predators, and alert people of your location if need be.
Lastly, here are some stroke/dehydration symptoms to look out for, either in yourself or your partner:
http://www.umm.edu/non_trauma/dehyrat.htm
edit:
Also, if you can, hike out before your planned hike and stash water at points along the path. This way you have a back-up plan if you need to.
