This may mean nothing more than going over the reading list, browsing through the old tests, and deciding that you've got a good handle on the material and don't have to start studying until later. However, you want to avoid waiting until later, and then finding out you don't have enough time to prepare. Again, even if you think you've seen the material before, pick up a syllabus so you can decide whether taking the course or studying entirely on your own is best. There are lots of notes, handouts, and links on this page that you should find helpful. It also shows how we organized our summer to prepare for the test.
Study groups are great organizational and motivational tools.
I from 5 years ago. They do have office hours during the summer. If your background is shaky in some areas, you may find it helpful to sit in on the undergraduate prerequisites to core courses. This is a great way to get caught up on material and will force you to get ready far in advance. You don't want to fail twice, and seeing the material a second time, now that you have some perspective, can be immensely helpful.