you are probably aware that anything less than 700 will almost rule you out of the top b-schools...i have seen indians re-take the GMAT with 720!! There are so many indians applying. So, firstly, you need to set a higher target for a GMAT score.
looks like you have the rite study material...but if you are not a native english speaker, you will find it a little difficult...
Some tips -
1. Memorize common idioms, structures, etc. eg - "not only X but also Y"......neither nor,
2. Memorize commonly used parallelism - "Unlike X, novel did Y"
3. Start reading good english editorials...You don't have much time till end of august, but it should hopefully help you a. get into the habit of reading long passages and b. develop ideas for AWA :-)
4. certain items are [u]generally[/u] not acceptable on GMAT SC - "being" "if instead of whether", "like"
I have seen a trend in OG...they stick to certain types of parallelism, idioms, etc. Also, look at the OG SC preface...past perfect, present perfect, etc. This is the backbone of their question...but ofcourse, they disguise their questions so well that it's difficult to identify these under exam conditions.