One of the greatest tools which can help you increase your GMAT score is the mock papers that can be taken by the candidate. A number of organizations offer mock papers for the GMAT. The most trusted source is the GMAC papers themselves but being limited in number they can be an issue to some candidates. Many other groups like Princeton Review, Manhattan Prep, Kaplan, etc. all offer mock tests along with their reading material.
Before solving the mocks the candidate must develop initial competency in the subjects and focus on the weak areas based on the mock paper results. One other important thing to be kept in mind is to set up periodic review sessions of the papers to narrow down to the weak areas so that the candidate can focus on them.
Solving 2-3 mocks every week is a good idea to be in prime shape before the examination. Best of luck!