The short answer: build up your physical stamina.
For the long answer, read on:
There's a lot more correlation between your physical and your mental abilities than most people realize. Note how the bulk of the debriefs from high scorers (700's) very closely parallel the approach an typical athlete will take when training for a main event. This is no coincidence, as it has been repeatedly shown that increasing your physical performance and conditioning through running and better physical fitness improves a wide range of your mental abilities, everything from memory to focus and concentration.
To boost your mental-bulge, an approach could be as follows:
1. Start slow, get familiar with the question types, subjects, format, etc.
2. Begin by studying subject areas.
3. Progress to doing sample questions, related to the subject area you are reviewing.
4. Increase the number of sample questions you regularly attempt gradually, while keeping your average percentage of accuracy in a log.
5. Begin to mix question types on one particular day while maintaining the total number of questions practiced high.
6. Progress to sample tests and mixes of question types and ranges of difficulties.
7. Rock the GMAT.
In parallel to this, you can incorporate an exercise routine:
1. Start with small weights and warm-up cardio (steps, jogging, bike) for 30min - 60min each day. [2 weeks]
2. Move up to slightly more weight and more total time (60min weights, 15-20min light cardio). Shift muscle groups every day, rotate around all the major muscle groups in your body every week or so. [2 weeks]
3. Increase weight again, being to do more serious cardio (running). [2 weeks]
4. Begin to shift your workout to cardio (30min weights, 30min cardio). [2 weeks]
5. As you get close to the exam, you likely won't have much time for daily gym, but it is still very important so: cut out the weight lifting or minimize to once or twice per week, and focus your attention on your treadmill: 30 mins at least 5 times each week topping out at 7mph at a minimum.
Following this routine will ensure that you have mental clarity and plenty of energy to tackle even the most daunting problems in the long stretch of the GMAT.