When writing the "analyze an argument" essay, please be warned that the GMAT Prep software has a bad reputation for swallowing up test-takers' essays--the software is a bit clunky, and for some reason, after you write and submit your essay in the software, the essay will disappear, never to be seen again. Thus, I strongly recommend that you write your essays on an outside word processor where your work can be saved as a backup, or at least make sure to cut-and-paste the text of your essay into a text document when you are done.
If you have a little extra money to spend (GMAT prep is not cheap), then you should also strongly consider purchasing the GMATPrep Exam Pack 1 and the GMATPrep Exam Pack 2 for an additional $50 each (or save 10 bucks by buying the Exam Pack Bundle). These contain four more official GMAT practice tests, giving you six real tests to work with instead of just two. While you're at it, you should also strongly consider buying the GMATPrep Question Pack 1 for an additional $30, which will give you plenty of questions you can practice on your computer, instead of just in a book (remember, the GMAT is a computer test and as such you should spend much of your time looking at questions on a screen). You can view all of those products in one place (now being rebranded as "GMAT Official Practice Exams and Questions") here.
Yes, these materials from the test maker are on the pricey side. However, Official GMAT tests are invaluable, because although there are some companies out there who write realistic imitations of GMAT questions, they can never quite match the idiosyncratic nature of real GMATs. Moreover, only the GMAC has access to the precise algorithms that determine your actual GMAT score. Tests from other companies are fine for practice, and necessary in many ways, but the only scores you can fully trust are these official tests from the test maker. If you do in fact purchase the Exam Pack 1, then you could take the 6 tests at evenly spaced intervals throughout your preparation, which would provide you with twice as much information about your progress. Avoid the temptation to take all of these tests early in your prep, for the reasons discussed above.
Pro tip: You can take each of the 6 GMAT Prep CATs more than once, because the GMAT is an adaptive test (it adjusts the difficulty level of later questions based on your previous responses). There are about 4 to 16 times as many questions in the GMAC's question pool as there are in any given test, which means that every test you take will be different. Tests 1 and 2 draw from a (gigantic!) pool of about 1,500 questions, and tests 3, 4, 5, and 6 draw from a more modest pool of about 400 questions each. To re-take your GMAT Prep tests, click "reset" in the lower-left hand corner of the GMAT Prep software window, but make sure to take screenshots of your previous test sessions beforehand--frequent screenshots are a good idea anyway because the software is prone to crashing and losing your data. For your screenshots, use either the "Print Screen" (Windows Key + PrtScn) button on a PC or (Shift + Command + 3) on a Mac. Thus, the 6 official CATs can easily turn into 12 or more CATs if necessary.
2) Purchase one, preferably two copies (the second copy should be left blank) of the 2018 Official Guide to the GMAT or--even better--the GMAT 2018 Official Guide Bundle, which includes the Quantitative and Verbal Review Guides (you can also read my exhaustive customer review of the Bundle, which is currently the first one on top). The Official Guide (OG) will become your new best friend: it contains over 900 practice problems and a diagnostic test, as well as free online access to copies of the questions through the (included) Wiley Online Question Bank. In addition, purchase a (print or digital) copy of Brandon Royal's Ace the GMAT: Master the GMAT in 40 Days, which will serve as your GMAT strategy guide and study plan. You will also find helpful free introductory materials on the web, on sites like GMAT Club and Economist GMAT.
Start working through these books from front to back. If you get stuck on something, then just mark the page and move on. The idea is to get yourself familiarized with the test as much as you can before you start taking a class or working with a private tutor. Don’t worry about timing yourself just yet, but you might want to record how long each question takes you. You get around 2 minutes per quantitative question, and 1.5 minutes per verbal question. In addition, you can use this modified GMAT Club spreadsheet to help organize your studies by topic, and for helpful links to question explanations.
As an alternative to the physical books, feel free to purchase the Kindle versions of the Official Guides--they are quick, convenient and environmentally friendly, and they work on nearly any device with a screen. For some reason, you must buy the books separately (not as a bundle) to access the Kindle versions. You can buy the Quant Guide separately here, and the Verbal Guide separately here.
Don't waste your time and money practicing on questions made by any other companies--these are merely inferior imitations of the real thing. If you must use other materials for test strategy, then that's fine, and in most cases necessary, but try your best to stick to official questions whenever possible.
3) Set a score goal. My students' GMAT student improvements are usually around 50-150 points, given time for sufficient preparation and dependent on the student’s starting score. To state the obvious: the higher your starting score, the fewer points improvement you can expect. IR and Essay improvements are more difficult to predict, but are usually around 1-2 points. Check out this chart that shows you how GMAT Math and Verbal scores combine to produce a composite score.
Working with a private tutor is the very best way to maximize your score, for a variety of reasons:
1) You are given personalized attention, lessons tailored to fit your schedule, and the opportunity to discuss each question in-depth until you are fully satisfied.
If it doesn't go well the first time, then remember that most students do better the second time they take the GMAT.
Update 7/16/15: if you do choose to cancel your score, then you no longer have to worry about the dreaded "C" showing up on your score report: the GMAT has discontinued this policy. You will also be allowed to retake the test after a 16-day time period, and you can take the GMAT up to five times per year.
The best way to prep for the GMAT is simple on its face, yet complex in its execution.