When I first started out I scored around a 580, and right now I am scoring around a 760. I hope this advice helps; I have written down what you absolutely need to do, what you should do, and what you can do as an extra option to help.
What you should absolutely do:
1. Purchase the official GMAC book
2. Set a date when you will take the exam and make a detailed schedule
3. Set several times throughout the week to practice, and set a date every study to take a practice exam (or part of one)
4. Take the Manhattan GMAT class (they are awesome!)
What you should do
1. Find a friend to study with; you two can keep motivating each other
2. Take a private tutor (they are expensive but they do help.)
3. Spend time in these forums taking practice questions
What you can do for extra help
1. Join the website 800Score for extra GMAT tests
2. Join the website 2minuteGMAT to be sent 10 questions each day
I hope this helps.