Ok, well, the first of my solutions may not be as relevant when the question was posed, as it is now!
1. We live in an age of distraction, of being connected all the time. Mails, notifications, alerts - there is an overload of information and the brain is always "busy" focused on something or the other. To defragment my mind, I have removed all social media apps from my phone. I know this will help, as I had experimented with turning off my phone for a couple of weeks . I saw a marked improvement in my focus, as the mind wasn't jumping all over the place.
2. A good diet. The body is a machine. The brain consumes a large chunk of energy, compared to it's mass. It is very important to eat healthy foods - lots of greens, nuts, fish, and to stay hydrated.
3. Exercise. It has been proven that physical activity helps retain new information. Go on small jogs regularly!
4. Rest. Don't push yourself to the limit daily.
5. Simulate the GMAT. All prep tests have to be taken in their entirety. This will train the brain. If I want to run 10k, I should practice running 12 to 14k, so that on race day, 10k will be a breeze