I passed the certification exam with 96.3% (77 out of 80 points). This is how I did it.

After almost 6 years of experience using Scrum as a developer (or at least what I tought that was Scrum) and three months of daily study, reading and preparation, I've got it, I passed the certification exam with 96.3% (77 out of 80 well answered questions): I'm a Professional Scrum Master I.

I followed all of the suggestions given by most of the users in this community to get it, so I'm just going to give a brief summary of those:

• Yes, you have to read the Scrum Guide once and again as much as you can; you have to UNDERSTAND (and even memorize) every single phrase and every single word and its meaning in the context of Scrum. In my case I read it at least twice a week. 
• Reading, understanding and memorizing Scrum roles, artifacts events and rules described in the Scrum Guide is not enough; you also need to understand how is Scrum implemented in real life in order to understand it deeply. The exam doesn't ask for implementation, but I think that knowing how is Scrum implemented helps you to understand better and deeply those concepts described in the Scrum Guide.

  A good source to understand Scrum implementation is the book Essential Scrum: A Practical Guide to the Most Popular Agil Process (Mike Cohn and Ron Reffries). It teaches you the scrum theory (based on the Scrum Guide) and complements it with real life examples and scenarios. In my opinion, the Scrum Guide only defines the What and the WHO, but this book explains the When and the Why, and those (mainly the Why) are specially important to answer properly many of the questions.

  Specially if you don't have experience by using Scrum, this source is a must in order to understand how is Scrum used in real life.

• Having experience using Scrum definitely helps a lot and is a great plus, but please, DO NOT trust your previous Scrum experience. I've been using "Scrum" (and "Scaled Scrum") for about six years in a really big development team in a big company, and while I was reading and preparing the certification, the more I learned, the more I realized that what we were using wasn't Scrum. Instead, I used my Scrum experience to realize and understand why we weren't doing things right according to the Scrum Guide and to figure out how should things had to be done to match the Scrum Theory.
• And, of course, knowing the Scrum theory, the Scrum implementation practices and having Scrum experience is not yet enough: You also need to know how to pass the exam. So, during all the preparation process, use the open assessments and the Mikhail Lapshin free exam simulators once and again. Don't wait until the end of your preparation; used them from the first day and every single day of it. It will help you to measure your progress and to discover your weaknesses. I used the PSM, the PPO and the Mikhail Lapshin assessments at least  once a day. I almost memorized every single question, but mostly, I understood WHY the right option was the right one but also WHY the wrong options weren't.
• In the exam, use your time wisely, you have less than a minute per question. Some questions are going to request more time than others, so try to save time on those pretty obvious quiestions, because others are going to be really tricky and ambiguous and will require you to analize them very well.    
• And, finally, the exam is really harder than the Scrum Guide, the books and the open assessments makes you think it is: When I finished my preparation I only got perfect scores (100) in the open assessments, and I got it so easily and so quickly that I tought it was going to be easy for me to finish the real exam in half of the time with a perfect score: Well...Nope! I got a good score (96.3%) and only missed three answers (out of 80), but I needed 56 minutes to do it (so I only had 4 minutes left to check all of the 10 questions I had marked to review) and, honestly, while I was taking the exam, all the confidence I had once, just dissappeared, and its place was taken by some fear and humility. Fortunately, having follow this path I just have described, worth the pay when I clicked on the finish button to realize that It wasn't that bad: I was a Professional Scrum Master I.

I hope my suggestions will help you as much as other users suggestions helped me to reach my goal. Without all of those suggestions and the help I found on this forum and on the Scrum.org page, I definitely wouldn't have been able to do it. So thanks to all of those who took the time to write their experiences here and thanks to Scrum.org for providing an amazing way to learn, prove and certificate, with your knowledge and not with your money, that you are a Professional Scrum Master.