it is hard to say what can you do to improve without knowing you. it is also difficult to say whether you need at all to improve. i personally don't think that the difference between 640 and 660 will make a big difference on your application, especially since they adcoms can see it is the 3rd time you take it. however - i'm not the right person to comment on that. i'm not applying for MBA so i don't know how adcoms actually make of such changes.
i also think that there is a potnetial "burn out" that you might experience. which may affect your performance (not in the correct side)...
so i'd think twice more before if you should give gmat a third chance or spend the effort and time in making other parts of the applications good (or just have fun....).
having said all that, and without knowing your weak and strong points - i'd say that DS in the quant and CR in the verbal are areas where improvements can be made without too much studying.
these are the areas where most make "unnecesssary mistakes". and my guess is that these questions are responsible for the quant score drop between your gmat1 and gmat2.
i can't say much about the verbal part as i'm no expert on this. on the quant part you probably don't need to study more math. you just have to practice carefully, paying attention to underlying logic and possible traps.
if you have specific questions on math... just ask, i'll be happy to help.
good luck anyway...