GMAT Prep tests are good indicators of where you stand in GMAT. For 600, do the concepts well. If you are clear on the concepts, 600 should be a piece of cake. Do the OG thoroughly.

thank you for the response, can you please elaborate a little bit on what you mean by "concepts"?

Karan is correct. Having the "concepts" especially in SC is very important. I don't think that you will be actually able to improve your score a lot by just taking n number of tests because you will probably make the same mistakes again and again. The best strategy is to go through a particular concept,then do sufficient number of questions which test that particular concept so that the concept seeps in your sub conscious mind . If you take the tests after that,then you'll see a definite improvement in your score.

Verbal is very tricky on the GMAT. You should try to really incorporate your GMAT Prep into your daily life. I did this during the months leading up to my GMAT and it's fairly easy to do and can really help you.

Whether you're watching TV, talking to a friend or listening to the radio, take every sentence/argument apart.

"The New England Patriots beat the Jets tonight, when asked whether 'they'..." Ambiguous pronoun 'they'.
Others might think that this is taking it too far or becoming obsessive, but it really helped me keep the concepts in mind and learn to find errors quickly within arguments/sentences.

Good Luck!