I did a number of things to study for the master exam (including taking a correspondence course and buying a code question CD). By far, the best thing you can do to get ready for the test is to read the codebook (or better yet, the handbook)! It takes a while and is about as much fun as reading the dictionary but it is the best way to become familiar with how to find things in and use the codebook. You could spend forever just learning facts but if you don't know how to look things up, passing the test will be hard. In my opinion, the people that spend big money on some type of master training program are just trying to find an easy way out.
Thus, start on page one of the codebook and start reading and taking notes. It is nice to know what the questions will be like so you may also want to purchase a code Q&A type disc.