Before we get into what food to cook, you need to be sure that you're getting enough water.  At About.com they have a handy hydration calculator to tell you how much water you should be drinking per day.

 

When you wake up in the morning, the first think you should do after taking care of necessary business, is drinking water.  Try to get in a liter right then, and drink the rest over the course of the day.  It helps to always have some water with you.  If you need to drink 3 liters, then drink a liter in the morning, and carry around a 2 liter bottle for the rest of the day.