When I took my first diagnostic test I scored a 141. After taking a Princeton Review course and studying my ass off for 3 and a half months I got a 160, not a harvard score, but a decent score. If you start now, and study every practice test to the point that you remember the questions, you should do just fine. STUDY, STUDY, STUDY and you will be fine. It is all about understanding the test, if you start to understand how the questions work, the rest of the test becomes easier. Oh, and learn how to manage your time.
I also wanted to say that the princeton review books and other lsat prep books are not the same. They sometimes may give you contradictory advice, if you read both.