It is somewhat amazing how scores and percentile rankings have changed over the years.
I took the GMAT in October, 1981 scoring:
Verbal 40 (93%) Would be 88% now.
Quant 39 (91%) Would be 48% now. WOW!!
Total 640 (95%) Would be 650/78% now.
The best advice I have for GMAT candidates is to start with a good study guide and see how you do on the practice tests it presents.
If you still need improvement go for a good online or live classroom study course.
A major benefit from the study guide was just getting familiar with the format of the exam. My main weaknesses were in Data Sufficiency and Critical Reasoning, but that was mostly from lack of familiarity with what the exam was looking for rather than from not being able to arrive at the correct answer. Once I figured out what the test was really looking for the rest was easy.
Good luck in your quest.