A good GRE score typically falls within the 50th to 99th percentile range, or between 151 and 170 on the two sections. The definition of strong GRE scores varies based on program.
A good GRE score is one that gets you into the graduate program of choice. Each program requires a different GRE score for admission, so it's important to do research before you apply so that you know what score is acceptable.
The best way to increase a GRE score is through studying and prep time. In doing so, it can improve the grasp on content, which will shine through on test day.