I did it about 10 years ago. I won't join a group but here are a few ideas:

- I didn't join a course, I just bought some preparation books. This may or may not work for you; it requires a lot of discipline. 

- Get these books on Amazon, and get several, especially those with practice questions.

- Even if you'll do a course, still get some additional practice material so you can practice whenever you have a few minutes. The exercises need to become second nature to you. 

- I started 2 months before the test, and worked on it (almost) daily (on the train ride to work, while watching TV in the evening, etc.).  

- As a starting point, make sure you understand the sections and type of questions that you'll encounter

- Brush up on your math and (if necessary) your English language. 

- Practice a lot of questions. After you've gone through a book of practice questions, restart. 

- The scores you get for these practice exercises are not reliable, because the real GMAT tests adapts to how you are doing (i.e. if you're doing well the questions become harder). Treat them as a way to diagnose your weak points.

- Math questions: if you feel that you need to do a lot of calculation, reread the question. Most questions have a 'shortcut', and the only way to score well is to look for these shortcuts all the time. 

Good idea