OG11 and practice, practice, practice on the real test. Some people will say that taking the GMATPrep tests more than once each has a diminishing rate of return because of repeat questions, but I think the value of getting comfortable with the testing format far outweighs that. The only other thing I used was the GMAT Focus quant prep from GMAC. Short and a little pricey, but it definitely helped me play to my strengths.
Getting your best possible score on the GMAT is all about knowing your own Q/V strengths and weaknesses. When I hit a type of question I was uncomfortable with, I made a quick, educated guess and moved right along to questions where the extra time may actually help. It worked out pretty well. I think for some people, hubris intervenes, hence the 5:00 questions. Just my opinion.
Lastly, I would take a practice test cold just so you know what you're in for. I did well on the SATs, but the GMAT was a huge eye-opener. If you start with a crap score, you can chalk it up to inexperience and you can just focus you're testing efforts on timing. If you study for three months then underperform, it's going to be a little more discouraging, even though it may all have to do with not knowing the testing format.
FYI -- if you go to a Vandy Discover Weekend, they give out the OG11. Great school visit too.