We recommend doing two things to prepare for the olympiad (besides, of course, attending our meetings):

The Feynman Lectures

The Feynman Lectures on Physics by Richard Feynman describe the whole of introductory physics with the voice of one of the most creative physicists of the 20th century. Feynman describes many concepts in innovative ways which help you understand them better, especially if you already know the conventional way of describing them.

Volume 1 is the cleverest volume, and the one which covers most olympiad topics. The TJ library has at least one copy; if you can't find it for free, though, it's worth buying.
Or you can borrow them from someone---Dr. Dell's got them, and people that have been to camp might have ended up getting a second/third copy for free, so asking a such person would also work.

You can't read the whole thing properly between now and the olympiad, but flipping to interesting-looking material and reading a chapter or two works well. In fact, you might also do the same with your textbook.

Former Exams

Get some past qualifying tests. We'll use the earlier exams for our practices, so please start from the most recent. Try them under test conditions -- timed, in a quiet spot with no distractions. Then comes the important part.

What you really want to study is the material that's on the contest, but that you don't know. So look over the problems that you got wrong or didn't understand, find those topics in a physics textbook and/or the Feynman Lectures, and read. You should probably spend considerably more time reading material related to the missed problems than doing the contest in the first place, then perhaps move on to another contest.