In order to get your 600+, you should targer higher than that. Let's say 650 plus.
Buy the
OG, 11th edition, do all the questions step by step. Keep an error logs. Also, check the explanations of the questions that you got right too so you can see the logic of the questions. I know it is a very boring process. If you lack confidence, do the
OG more than one time. Remember it is very important to track your mistakes and keep on working on them. It is the only way to develop the GMAT thinking process
If you did the GMAT twice, you already know that the GMAT is very similar to the
OG, except that the questions that you will counter are questions that have never been seen before, but they do represent the same logic than the questions seen in the
OG.
Keep in mind that
speed is very important. ETS like to see how people react under stress, how do they manage their time during the exam, they also test self confidence level. Some people will get stressed when they face tough questions and they lose control of the exam.
Keep this attitude and you will get the score that you want. If not at least a score that is very close to your target score. At the end of the day, remember that the GMAT is only a very stressfull exam. Take it as a challenge and an opportunity to learn new stuff and you will definetly take an advantage of it, even if you do not get your target score.