Take regular breaks. Your brain can only take so much in one sitting. When your mind starts to wander, or you find yourself daydreaming or reading the same paragraph over and over again, it’s time to take a break. Spend 10 minutes away from the books, but don’t enter another activity that requires concentration or would result in distraction. In other words, don’t check facebook, don’t visit with friends, do read email or rss feeds. Instead, take the opportunity to go to the bathroom, drink a bit of water (500mL is good), and stretch your back, neck, arms and legs.