In essence, you should try to get a basic grasp what doing research is all about, first. And second, you should try to get a basic grasp about the who and what is important in your area of interest.
Now, these are broad topics and something you can't expect to learn like a lesson, it is more a journey that will ease some important details - of the kind that no one will (or can) tell you about and are nowhere to be found written out - that time spent in academia will bring along.
For the first part, follow some academic blogs, professors and other academics write a lot about the everyday experiences of their research programs. Even if their field is not your own, try to read between the lines and draw yourself a bigger picture. Although the day of a researcher differs by the field they are active in, you will find many common traits. Getting the experience of other people about paper writing, research methodology, thesis committees, and similar topics was of tremendous help to me, before I started with my graduate studies.
The second part includes narrowing in to you specific area of interest. Use your time to find currently hot research that is done. Identify the people who do it and visit their websites to check out their work. Try reading the published work, don't mind if you don't get all of it. Focus on the problem at hand and the proposed solution. Even if you only understand the abstract, introduction and conclusion it is enough for starters. Also, pay attention to the group of people at the institution where you will be doing your MSc. What are the topics of their research? Do they have labs / what projects are they working on? What classes are they teaching? Is some professor/topic/project of particular interest to you?
Finally, I wouldn't recommend burdening yourself with an over extensive amount of prep-work for classes or thesis. Of course, feel free to explore, experiment, etc. but the majority of this will come naturally once you have an adviser, classes, a group, and so on. If you find that a skill is demand in your field, try allocating time for getting the basics of it (e.g. if you never have written anything in Latex, try it out).
As you see, my recommendations are primarily focused on getting to know the world you are getting into. This proved far more important to me than preparation for the coursework and thesis. Also, it helps you solidify you interests in research and puts you in some (real) perspective what you can expect from graduate studies.
