Water intake before the trek is just one of the factors that allow one drink a little bit less during the first day; more importantly, the same factors let you start the trek with much more energy and tolerance to discomfort. That will eventually help you combat thirst as well, provided that you still don't expect miracles from your preparation and still carry enough water with you - basically as usual.
Eat foods rich in polysaccharides (pasta, bread, vegetables,...) for the last 1-2 meals the day before the trek. Glucose is stored in your body in the form of glycogen, and glycogen binds a lot of water. One to four, or you could say that 80% of what's labelled glycogen in your muscles and in your liver is actually water, invisible to your hydration control mechanisms. However, it takes time to accummulate glycogen and throughout that time the supply of glucose needs to stay away from both extremes; polysaccharides make that easier because they are digested slowly (compared to sugar) and they release glucose into your bloodstream gradually.
While you are doing so, follow the usual hydration advice as well; without sufficient (slow, gradual) water intake you will just not be able to build your glycogen reserves either.
Get enough sleep. I'm getting boring, right?
Granted, we are not talking about huge quantities of glycogen and water accummulated. But any form of physical exercise is just plain more enjoyable and healthier if you start it hydrated and full of energy. And learning to eat and drink properly is a life skill that only gets more and more useful as you will age.