For a safe hike, drink lots of water. Dehydration is caused when you fail to replenish your fluids. Symptoms include dark urine, mental confusion, headache, nausea, dizziness, and lack of coordination. When you become dehydrated you may not realize your state, making matters worse.
     The best way to beat dehydration is to drink lots of fluids, like water, or sports drinks, but not caffeinated beverages (they are diuretic and will cause you to loose more liquid). Since it seems there are no longer any pure mountain streams, you must either carry water purification equipment, or bring water or other beverages from home with you. It's not a good idea to count on water being available at the trailhead, as you need to make sure you will have water.
     Sometimes when you are hiking you may be having such a good time that you won't feel like stopping, pulling out the water, and drinking. It's vital that you do. A good rule is to try for a few deep swallows of water every 20 minutes or so. If you feel thirsty, it's already too late; you have started to become dehydrated. A good motivation for drinking the water you carry with you is that the more you drink, the less you have to carry (don't think about it too carefully and it may work for you).
     In 2001 I finally decided to try a pouch hydration system, after overcoming a suspicion about my ability to keep one clean. The newer products have zip-lock tops for easy sanitizing, and are lighter than Nalgene bottles. I am a convert. My water consumption has risen dramatically, and so has my endurance level. With a bite valve clipped to my backpack strap, I no longer take water breaks, I just frequently drink. On hot days before I leave the house I throw a few handfuls of ice into the pouch, and this keeps the water cool all the way to the end of my hike. The only negative is the thermal property of the plastic tubing. Since water sits in the tube, it gets warmed by the sun. If you don't clear the tube frequently, the water can get bathtub-temp warm.