Done with GMAT finally - scored 720/5.5 !

for SC, start with basics, go through OG 12 problems carefully. note down the strategies mentioned in the solution. Also Manhattan SC guide is a must. Read it and understand the concepts. its important to understand why an answer is wrong,and other is right. This will help minimizing mistakes.

For RC : practice more. Read more and more. Do a lot of RC practice, day in and day out. You cant master RC without doing a lot of practice. You should reach at a stage when you are not doing any mistakes in any passages, or may be one odd mistake in 10.

Do not panic. Take a lot of mock tests, and under test environment.Don't let anything disturb you. Take the GMATPrep tests seriously. And try to concentrate on the question at hand, not at the questions you already answered.You cant go back anyways.

I would suggest that since you have already taken the test thrice, take some time this time around, and postpone the date if you think you are still under prepared.