Failure to drink adequate fluids leads to dehydration. Dehydration can lead to fatigue, headaches, and even muscle cramps. Even if you don't have any of these symptoms, you still need to be sure that you get adequate fluids. This is especially important during the August practices and the early games in the season.
This year the American College of sports Medicine (ACM) released advise on fluid intake for health and performance.

The day before practice or a game, eat well and drink plenty of fluids Drink more fluids than you think you need. Thirst is not a good indication of needing fluids.
Drink fluids before practice or a game. During practice or during a game, drink plenty of fluids. Be sure to drink at least one glass of water or sports drink at half time. Remember to drink at half time even if you are not thirsty. Thirst is NOT a good indication of need for fluids.
Keep your beverage cool. Studies show that cooled beverages are more platable and that you will tend to drink more.
Try to drink a sports drink during games--especially the first game of the day. Studies show that if exercise lasts over an hour (a soccer game is 90 minutes for U18), a drink with electrolytes and carbohydrates is better than plain water. For shorter periods of exercise, water is just as good as sports drink. Avoid drinks that are too high in electrolytes or carbohydrates.
Consume carbohydrates at the rate of 120 to 240 calories per hour during exercise lasting over an hour. This will delay fatigue. Drink 5 to 12 ounces of a typical sports drink every 15 to 20 minutes during a game to meet both fluid and carbohydrate needs.
Sports drinks with some sodium are recommended for exercise lasting over an hour.

If you suffer from cramps, drink several glasses of sports drink the day before a game or practice.

copyright @ 1996 Les Sparks.