All the health information I read says that we need to drink eight glasses of water a day. In summer heat or following sweaty exercise, you may need a lot more. A rule of thumb is to drink an extra 16 to 32 ounces of water for every hour of exertion. We talk in terms of water because it's the cheapest and, in general, the best choice. Decaffeinated fluids are fine.
Caffeine has a mild diuretic effect.
Sport drinks, which replace electrolytes and have extra carbohydrates, are helpful if you've lost a lot of water. They're especially useful if you're sick and have lost fluid from fever, diarrhea or vomiting. Avoid alcoholic beverages to rehydrate you, since they dehydrate you by encouraging urination. Orange juice is a good energy drink, but dilute it. Too much sugar in a drink will slow down the rate at which water is absorbed.