Here are some important things to remember:
A 150-pound person requires about two-and-a-half quarts of water a day for general physical maintenance during normal activity. If you are planning on a day of long approaches and multiple pitches, double that amount. Remember that drinking before and after a climb counts; don't drag half a dozen quarts of water up a route. The extra energy you burn hauling the load will just increase the amount of water you need to drink - a perfect Catch-22.
Drink fluids before you get thirsty. A loss of only 2% of your body weight through fluid loss (fluids make up 60% or more of our weight) can have a detrimental impact on performance. A good way to monitor your level of hydration is to watch your urine. If it is clear, you have nothing to fear. Yellow urine is a sign of dehydration. Even worse is when you are drinking and not urinating, because your body is utilizing all of the liquid you are consuming. In both cases, drink more. Another sign of dehydration is a headache, accompanied by a sick feeling and lack of thirst. In this case, force yourself to drink fluids.
Humidity is your enemy. On hot, dry days, sweat is quickly evaporated from your skin, which serves to cool down your body. On humid days, the air can contain nearly as much fluid as your sweat, which inhibits your body's ability to dissipate heat. The heat you generate through exercise remains trapped in your sweat and tissues, thus raising your body temperature. In an effort to cool off, your body produces an excessive amount of sweat, leading to dehydration. The answer? Drink more water.
Dress in layers. Bring along a raincoat, a fleece jacket, and a T-shirt for when the sun is beating down on your back. A lightweight, long-sleeved T-shirt is a good choice. If you are overdressed, heat will be trapped under your clothes, hindering evaporation, causing your body to work overtime to stay cool. You'll also sweat more, and lose fluid more quickly when you're overdressed.
Altitude can have a negative effect on hydration. People tend to breathe faster when the air is thinner; you actually lose more fluid through evaporation in your respiratory tract. Your body will need more fluids at altitude than at sea level, not just for hydration, but to help stave off the effects of altitude sickness, so plan accordingly.
Plain water is fine for hydration, especially if you are maintaining a low-to-moderate aerobic intensity for an hour or two. Sport drinks aren't a necessity since a day of climbing isn't usually sufficient to lower your amino acids, blood sodium or cause an electrolyte imbalance. Most sport drinks have salt, some sugar, and plenty of carbohydrates, so they can provide some quick energy if you forget to eat lunch. But the truth is that plain water will rehydrate your body more quickly than liquids with added electrolytes or salt. The best rule of thumb, however, is to bring along something you'll drink. I have one friend who humps in cans of Diet Coke to the base of the crag. Weight aside, it is probably not the optimal sports drink, but he's always outperformed me on climbs with no visible ill effects resulting from his drinking habits.