Drink 8 oz of water 2 hours before exercise (including short/easy sessions).
If you suspect you are dehydrated (statistically speaking, you probably are) - AND if you are going to have a long, hard, hot session - try Skratch Labs Hyper Hydration or equivalent 1-2 hours before exercise, paired with lots of water!
Hydration begins 2-3 days before a big event, especially if you are traveling.
If tapering or leading up to an event, stay hydrated with water at rest and sports drinks while exercising. Remember, salt and potassium come from food too! Wait to supplement these electrolytes until a couple hours before a race or event.
All of the above can maximize your performance, heart and vascular function, and body temperature in the right context! Play around, test it out, and see for yourself how it feels!