Do research. Get a stellar recommendation letter from a professor. Grades should be As and Bs, but matter much less than your recommendation letters and research. If you are from a less well-known school, your grades and your GRE scores should be very good; if you are in a top-ranked program then they are not as important. If the professor writing your recommendation letter is well-known, and the people reading his recommendation will recognize his name and trust his opinions, then the recommendation letter will be especially helpful. Best of luck.