Last week, I took the GMAT for the first time and got a 620. I had studied (consciously) for about one month and never took a practice test. After taking the test, I felt that I could have done much better if I hadn't let my nerves get the best of me, but at the same time I realized that I have what it takes to score a 700 or more on the test, so in a sense I am glad I didn't get the 650 I needed to apply to the b-school I am aiming at. I am now retaking the test on January 28 (I am applying to schools in europe, so the deadlines are different), but I don't know what study approach I should take to significantly improve my score in one month. I have been studying with the Manhattan Guides, and the OG. Does anybody have any advice?

My score report from my first attempt:
Quantitative 39 (48 percentile)
Verbal 35 (74 Percentile)
I should point out that I am not a native english speaker and that I feel that timing was an issue (I left the last 3 questions on quantitative section unanswered).

I would really appreciate any suggestions concerning a study plan or other tips.

Thanks!