Over the years I have developed the habit of reaching for the water bottle (20 oz size) every ten minutes. How much I drink each time depends on temperature. This approach allows me to keep hydrated without thinking much about it. When the ride is planned as longer than two hours, the second water bottle is filled with an energy/electrolyte replacement drink. Given I usually ride with my cycle computer display showing elapsed time, the ten minute monitoring isn't hard.
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A conclusion is the place where you got tired of thinking. - S. Wright
Favorite rides in the stable: Indy Fab CJ Ti - Colnago MXL - S-Works Roubaix - Habanero Team Issue - Jamis Eclipse carbon/831