Drink water even when you are not thirsty. Being thirsty is the most obvious sign of dehydration, which is in turn the most common cause of headaches.

Neuroscientists report that babies have the most water in their bodies, being born at about 78%. By one year of age, that amount drops to about 60 to 65% for men and 50 to 60% for women. The human brain is about 85% water; quite a high percentage. Thus drinking water helps you think better, be more alert and more concentrated.
The bones are between 10 to 15% water. Proper hydration helps keep your joints and muscles lubricated, lowering your chances of getting cramps and sprains. Your liver needs fresh pure water to purify your body. Water helps to replenish skin tissues, moisturizes skin and increases skin elasticity. So you definitely look younger when your skin is properly hydrated. Water helps you to maintain body temperature, metabolize body fat, aids in digestion, lubricates and cushions organs, transports nutrients and flushes toxins from your body.
Water is used by the body to help flush out toxins and waste products. If you do not drink enough of it, your kidneys are overworked. The colon has problems moving your bowels, then your body retains the unhealthy toxins it takes in. Your body reabsorbs the toxins back into your bloodstream which are then circulated around your body, into your body organs. This leads to another new set of problems. The heart, for instance, will need to work harder to pump out the oxygenated blood to all cells. At some point it will be exhausted. Other vital organs will be exhausted as well and so will you.

If you are drinking a lot of water and still feel thirsty, you might want to check your blood sugar level.
They say that too much of something is dangerous. Drinking too much water causes water intoxication. Rapid intake of too much water is also dangerous. Instead of taking all you need in one gulp, spread your intake throughout the day.
Do not count your caffeine and alcohol intake into your recommended 6 to 8 glasses of water per day, as these two are known to be notorious dehydrants.