Hi hugdor,
From what you've described, you haven't yet put in the necessary effort to score higher than you did, so there's some question as to why you would take the GMAT now (instead of pushing it back). Before I can offer you any advice about your studies, I'd like to know more about the work you've done so far and your goals:
1) How much of the last 6 months did you really spend on your studies? How many hours did you study during a typical week?
2) What is your goal score?
3) When are you planning to apply to Business School?
Thankfully, the GMAT is a consistent, predictable Test, so you CAN train to score at a higher level. You will have to COMMIT to the process though.
GMAT assassins aren't born, they're made,
Rich