Ok, I didn't have this kind of drastic improvement but I have taught people who have, it's perfectly feasible. The basic thing that this exam tests is your reasoning ability (verbal and analytical) and to prepare for it you have to train like you would train any other ability - intelligent practice. What that essentially means is figuring out/learning the best approach for each of the different question types, practicing the approaches until they become second nature and you can do the questions with reasonable accuracy, and then doing practice tests under test conditions. There's more to it than that but those are the essentials. And it takes time.
In terms of preparing for it, you have several options that, depending on your specific situation, any one of which can be useful.
The first is getting a tutor. In general this is the best approach since you'll get individual attention from someone who's, ideally, experienced and competent. The downside is that it's of course more expensive. But a good tutor will know the test, will have mastered the strategies for doing well on it, and will be a good teacher - they'll be able to communicate all that to you in a way that makes sense. Obviously I think my company
is the best or else I wouldn't be doing this, but getting private tutors from the major companies or from someone on craigslist/etc can work out as well. Of the prep companies the ones that are the market leaders in terms of quality, in my opinion, are Testmasters and Powerscore, followed by Kaplan at a distant third. If going for a private tutor what you need to be looking for is someone with experience and teaching ability. I can't stress enough that just getting in the 99%ile doesn't make someone a good teacher. Don't bother with people who don't have introductory sessions where they explain their approach and what you'll be doing and who don't have a free lesson.
The second option is a prep course. If you go with the big national companies, the ones I mentioned above are considered the best. There might be small local companies where you're at that offer a quality product. Check in with them and ask for details, referrals, etc. Prep courses are cheaper than tutors with the downside being lack of individual attention. That may not necessarily be a problem, it depends on your learning style.
The third is studying on your own. Though this is generally discouraged it can have very positive results for certain people who have an independent attitude and confidence in their abilities. To study on your own the best way is to find strong prep material and work from it. And then do as many prep tests under test conditions as possible. Once again, I think my material
is the best but in terms of the books out there the Powerscore books are by far the highest-quality. Though I personally don't recommend their approach to logic games - a better approach in my opinion is found in a book called "The Big Fat Genius Guide to Logic Games". And then I'd recommend buying all of the prep tests and having all of them done at least a week before the actual test.
Hope that helps, let me know if you have any questions.