Can I ask why you're targeting a 780? There are a few good reasons to target such a score, but none of them are about admissions - I just want to make sure you know that. There's no school that's going to say, "Oh, this student got a 780 instead of a 740; we'll let him in now." They really don't let people IN based upon the GMAT score; it mostly just keeps people out if it's too low.
There are some other good reasons why you might want to go for that kind of score - maybe you know of a scholarship that bases part of its decision on the GMAT score, or an internship that is very competitive and requires an extremely high score. Maybe you want to come work for us while you're in business school. :) Finally, maybe it's just an issue of personal satisfaction / achieving a personal goal.
I'm asking because I want to make sure that you know what you are and aren't getting out of the time you'll have to spend to try to achieve that goal. And I also want to make sure that, if your ultimate goal is really about admissions, you're spending your time in the best possible way. If you have 6 months, you could also use that time to: earn a promotion at work, take on extra responsibilities and a leadership role, develop a leadership role at a non-work organization (a charity or something like that), etc. If you do score in the low 700s (or higher) on the real thing, then those other things are more likely to bolster your application than upping your GMAT score.
Okay, so having said that, start here:
http://www.manhattangmat.com/articles/7 ... erence.cfmThis article explains the major differences between people scoring around 700 and people scoring around 760 (99th percentile). This will give you an idea of what you have to do.
How to do it? Your timing has to be nearly perfect:
http://www.beatthegmat.com/mba/2009/12/ ... managementYou have to be able to build your recognition skills to an elite level (as discussed in the above 700/760 article); here's how to analyze problems in order to do that:
General article:
http://www.manhattangmat.com/articles/a ... roblem.cfmExamples of above, one per major question type:
http://www.manhattangmat.com/articles/GMATprep-SC.cfmhttp://www.manhattangmat.com/articles/CR-assumption.cfmhttp://www.manhattangmat.com/articles/a ... estion.cfmhttp://www.manhattangmat.com/articles/a ... roblem.cfmhttp://www.manhattangmat.com/articles/a ... roblem.cfmDecoding quant problems:
http://www.beatthegmat.com/mba/2010/09/ ... t-problemsYou have to make very few careless errors (that is, missing things that you do know how to do):
http://www.manhattangmat.com/articles/error-log.cfmYou will still have incorrect answers, of course, because the test will still give you things you don't know how to do, but you can only afford a couple of
careless errors.
You may also need to go through our archives of articles to pull out topics that you need in particular:
http://www.manhattangmat.com/resources/http://www.beatthegmat.com/mba/manhattan-gmatBasically, you can't have any weaknesses at the 780 level, so you'll have to go through the 100+ articles to find additional help on the specific areas where you're struggling.
Finally, you are going for a VERY ambitious goal. Many people (though not all) would need some outside help for that. You're already asking for help here - that's a good start - but you may also need to look into help from a class, workshop, or tutor. You DON'T want to do a regular class - most people are trying to get to 700, so if you're already there, you'll be bored. There are some classes and workshops targeted at higher levels, though - and a tutor, of course, would customize directly to you (though a tutor is a lot more expensive).