Master AdWords. Don't just read the study material, yes there's some specific questions on the exam in the study materical, but the bulk of the exam is base on your understanding on how AdWords work. I read the study guide several times on my commute to work every day. I found that the day-to-day knowledge to be more useful.
This is a multiple choice exam, and pretty much every questions has one really bad "throw away" trick answer.
The exam takes about 2 hours, make sure that you leave plenty of time at the end to review any questions that you mark. If you spend more than 2 minutes per question, then you didn't read all the material carefully.
Watch some of the video demos in the study material, the questions/answer sections are helpful to see how much you know the material.
I found that taking notes while reading the study material makes it better to understand the material and makes it easy to commit to memory. The questions at the end of the 'Try it Now' were also very helpful to see how much I understood the material.
The 'Best Practices' were good source of some of the questions on the exam.
Don't ready third party study guides on using AdWords, they will distract you from Google's "best practices." This is Google's exam and they will focus on their own best practices.