The biggest thing that I have taken away from this article (and I have heard this before) is that if you are thirsty, you are already dehydrated.
If you weigh yourself naked before and after a workout of typically 1 hour and you are still within 2% of your starting weight, your hydration is on track. Too little water can cause dehydration, but too much can also after your performance and then you are not getting the maximum benefits of your workout. Everybody is different, a runner will use more than someone like myself who rides a stationary bike for a shorter period of time. Weather can also affect the need for water. Hot and humid weather will cause a body to sweat much more than a cooler Fall afternoon. Using the 2% rule and adjusting your water intake during exercise should keep me on track.