You know how they used to say that we should drink 8 glasses of water a day ... yeah, well ... that's not bad advice. You've mentioned what you drink when you ride (not a lot), but how much do you drink during the day?
Try for 8 glasses of water a day ... and then also, in addition to that, when you ride, aim for one 750 ml bottle every 1 to 1.5 hours.