Don't you love a good discussion?
More and more I see the statement, "By the time you feel thirsty, you are already dehydrated." That sounds pretty ominous. While it's technically true, the message it gives is usually blown way out of proportion.
From Principles of Anatomy and Physiology, 10th edition, Tortora and Grabowski; "When water loss is greater than water gain, dehydration - a decrease in volume and an increase in osmolarity of body fluids - stimulates thirst." Pfredricks and others are correct in saying you may feel different levels of thirst. That's because greater dehydration results in a stronger thirst level. It's like the body saying, "Hey, I said I'm thirsty!" By drinking before you feel thirsty, you are by definition over hydrating yourself. The body can handle that as long as you make sure you replenish your salts. Also, the plants will appreciate the extra nitrogen.
Otherwise it's common sense. If you're on the trail and you feel thirsty, drink. If you get hot, stop and rest in the shade. Eat salty foods or drink something that will replenish your electolytes.
This isn't rocket science and the human body has a great ability to maintain itself and let you know if you need something. Humans have skipped about on two feet for a long time before gookinade was invented. One thing they did have that we have lost is activity. A fit body does much better in the heat than an unfit one. So get out, take a hike, know your limits, and have fun.
"Sometimes I wonder whether the world is being run by smart people who are putting us on or by imbeciles who really mean it."
~ Mark Twain