I recently took the GMAT and got a 780, a 120 point improvement from my first practice test. I’ve definitely found the advice on this message board useful so I feel obligated to share my experience and what worked for me. First, the OG is a rather obvious must. Second, the Manhattan guides are EXTREMELY helpful – I ordered the Sentence Correction, Geometry, Equations/Inequalities/VICs, Word Translations, Number Properties and the Fractions/Decimals/Percents, and, with the exception of the latter guide, found them to be my most useful resource. Third, the online practice tests that you can download after you’ve registered for the GMAT are EXACTLY like the real thing – you should definitely take both. Fourth, Kaplan’s GMAT800 was a good source of difficult questions but I don’t think it helped all that much at the end of the day (questions seemed too different from the real thing). I’d recommend it only if you’re running low on questions from the OG and its supplements. Finally, and I don’t know that this has any practical value, but I was absolutely convinced that I would have to cancel my score two questions into the quantitative section (I wasted 15 minutes on them). I guess my point is that sometimes you can be your toughest critic and you should think twice before canceling a score.
Best of luck to all who have yet to take the test!