You can prepare with GMAT prep books, traditional classroom courses, accelerated and crash courses, online courses and private tutors (more about GMAT prep beyond study guides).
You can spend a great deal of time and money preparing for the GMAT. Which approach works best for you?
To figure out your own GMAT test prep strategy, consider such factors as your:
For example, do you learn best with the structure and group environment of a classroom? Is your workplace conducive to online courses?
For GMAT test takers who have been out of school and working, you will need extra time just getting used to taking tests. This was true for me. I had not taken a test in many years and needed some time to get used to taking tests in general. I found the study guides helped me a lot here by giving me explanations and lots of practice.
Keep in mind that you can use multiple ways to prepare for the GMAT, not just one.
The most important action towards improving your GMAT score is to practice, practice, practice. Practice increases your speed which improves your GMAT score.