I too experienced a significant drop for my first sitting. This is a telltale sign of burnout.
A reminder, my sittings and respective results were:
1. Oct '08: 155
2. Feb '09: 173
For October, I studied approximately 2 months (7-8 weeks) and was averaging 173-175 on my best PTs from a diagnostic score of 151. However, in the remaining 2 weeks before test day my scores dwindled; first into the upper 160s, which made me ever more anxious; then into the low 160s, which still functioned to make me ever more anxious, then again into the upper 150s.
The problem(s): Firstly, I was taking older PTs and had yet to acclimatize to the newer (post-2005) test format. Secondly, I was just throwing PTs at myself, nearly 4 a week in some weeks with minimal review. Moreover, the more anxious I felt, the more PTs I self-administered. Finally, I hated the LSAT and I became increasingly pessimistic in the world as test day drew nearer.
For my February sitting, I reviewed the Bibles at a leisurely pace, and took just 1 timed diagnostic prior to my sitting (Dec 2008) on which I scored a 177.
The Lesson: Dont get stuck on a score; and more importantly, TAKE A BREAK. I dont know how many times I've espoused that advice to people on this forum; I'm not joking. Relax, sit back and try to gain some perspective.
Ideally, you want to peak on test day; not be so sick and tired of the pages of the LSAT that you tune out when it counts.