All professional footballers, including David Beckham, know how important it is to keep hydrated. Hydration is a measure of how much water you have in your body. Water is one of the most important nutrients in your diet. You can survive for only a few days without water, although you can live for weeks without food.
Drinking too little water or losing too much water through sweating will have a negative effect on your performance in training and matches. Effects on performance will mean that you:
run slower
don't cover as much distance
react to the ball slower when tackling/passing/intercepting/saving
can't jump as high to head or gather/clear the ball for goalkeepers
Exercise increases the production of heat from your body, and sweating helps you to lose this heat, keeping you cool and preventing you from overheating.
However, it's important to replace these sweat losses otherwise you will become dehydrated.
If you do become dehydrated, your body temperature rises, performance suffers and at the extreme you could suffer heat stress.
The simplest way to tell if you are adequately replacing sweat losses is to check the colour and quantity of your urine.
If your urine is very dark you need to drink more fluids. When your urine is pale yellow your body has returned to its normal water balance.
If your urine is very dark, you should drink 500ml of water immediately, and continue to drink until your urine is pale yellow again.
You can monitor your sweat loss by weighing yourself before and after training and matches. For every 1kg of weight that you lose, you should drink 1500ml of fluid (1.5 times what you have lost since you will pee some of this out).
In addition to monitoring urine and weight loss, you should also monitor how you feel. If you feel chronically fatigued, have a headache, or feel lethargic then you may be chronically dehydrated, and you should continue to drink until you start to feel better.
Thirst is a poor indicator of hydration status, since you are already dehydrated by the time the thirst mechanism kicks in. Young children especially have a poorly developed thirst mechanism, so you should make a big effort to drink before and during training and matches, and afterwards.
The safest way to monitor your hydration levels is through the colour and quantity of your urine as highlighted earlier.
In a typical training day you should aim to drink between 3 and 5 litres of fluid. The exact amount will depend on the air temperature, your body size, daily activities and how much training you do.
You should experiment by drinking different amounts and checking the colour of your urine.
Try to avoid drinking too many fizzy drinks since these may cause you to feel bloated and may also cause stomach upsets.
Also beware of the sugar content of some soft drinks, since these can be very high in calories. Most fruit juices also contain a lot of calories, so drink these in moderation or dilute them with water. Drink as much water as you can, or choose diluted fruit juice or low-sugar cordials, as the main volume of your fluid intake.
Beware of drinking too much tea and coffee due to the caffeine content, because this can cause you to become dehydrated. As a guide, try to drink a glass of water with every cup of tea or coffee you have.
Pre/during/post matches you should aim to drink a sports drink, since this will provide energy and electrolytes, such as sodium and potassium, which are lost in sweat. You should aim to practice drinking during training.
When you're training or playing matches, you should aim to drink:
at least 500ml (approximately 1 pint) one hour before you start
then drink 200ml (a typical glass) 15 to 20 minutes before you start. This fluid in your system will be ready to replace sweat losses.
aim to drink approximately 200ml of fluid every 15 to 20 minutes during training. In hot weather you may need to drink more than this, and less in cold weather if you're not sweating and the session isn't that intense.
try to drink as often as you can during matches. But you're dependent on stoppages in play, so place fluid bottles all the way round the pitch so that you don't have to come over to one point to get a drink.
Aim to drink during half-time.
drink immediately after the match to start replacing the sweat and energy you have lost during the match.
The FA advises professional footballers to avoid drinking any alcohol immediately following training and matches. This is because after training you will already be dehydrated, and alcohol will cause you to become even more dehydrated. This will mean your body will take longer to recover and to return to normal hydration levels. So at least make sure you replace any water you lost during your training or match before indulging in any social activities!
Drinking too little water or losing too much water through sweating will have a negative effect on your performance in training and matches. Effects on performance will mean that you:
Exercise increases the production of heat from your body, and sweating helps you to lose this heat, keeping you cool and preventing you from overheating.
You can monitor your sweat loss by weighing yourself before and after training and matches. For every 1kg of weight that you lose, you should drink 1500ml of fluid (1.5 times what you have lost since you will pee some of this out). 
You should experiment by drinking different amounts and checking the colour of your urine.
Beware of drinking too much tea and coffee due to the caffeine content, because this can cause you to become dehydrated. As a guide, try to drink a glass of water with every cup of tea or coffee you have. 
The FA advises professional footballers to avoid drinking any alcohol immediately following training and matches. This is because after training you will already be dehydrated, and alcohol will cause you to become even more dehydrated. This will mean your body will take longer to recover and to return to normal hydration levels. So at least make sure you replace any water you lost during your training or match before indulging in any social activities!