Hydration is the greatest determinant of strength. A drop of 1.5% in water levels translates in drop of 10% your maximal strength. The leaner you are, the worse it is. Make sure you weigh the same or more at the end of your training session. High water levels = more sets & reps= greater changes.
Here is how much you should drink:
0.6-0.7 ounces per pound of bodyweight
39 ml per kg of bodyweight.
Therefore, 120 to 140 ounces for a 200 lbs man.
3.55 to 4.14 liters for a 91 kg man.
A recent study showed that men drinking 5 glasses of water a day vs 2 glasses a day, had a 54% lower risk of dying of a heart attack.