if you don't have the motivation to just sit down and take some practice tests, what are you doing signing up for more years of school?People are motivated to do specific things, often because they enjoy them. I enjoyed my 1L course reading, so I did it all (okay, I didn't derive much pleasure from CivPro or Dersh's many books and essays, but the generalization is true). The LSAT does not test the law. It has little to do with what you learn in law school, even if it does accurately predict law school success or sort out people who lack the necessary analytic skills. I don't think a lack of motivation to do dozens of practice problems for the LSAT/GRE/etc. indicates that you're not cut out for graduate education.
I come down in the middle on the question of the utility of a course such as the Princeton Review. I signed up for the class and paid my $1000, but after one session and a diagnostic test I realized that I was not the target audience for this class. However, I always do very well on standardized tests, and for someone who was trying to get from, say, 145 to 160, I think such a course could be beneficial. I cut my losses and got nearly all my money back. However, I did get to keep the course materials. I think these were worth the non-refundable 200-300 bucks.
My two cents: you can overstudy on the LSAT. I know someone who did; she obsessed over it for more than a year and then ended up scoring lower on test day than she had in some of her first practice tests because she was burned out. You can also under-study. I was cocky going into the first online diagnostic I took junior year. The unfamiliar format and types of questions threw me for such a loop that I spent a significant amount of time considering a career in library science. But after some additional practice, my scores came up to the percentile level that is typical for me on standardized exams.
My suggestions would be to take a class if you have practiced a bit and don't seem to be making headway at increasing your score. You might also consider it if you do really badly in one section (like games) or would have trouble keeping yourself to a study schedule on your own. It is not necessary to take a class, though: working through 2 practice exams (use real tests!) per week for a couple of months prior to the LSAT should get you accustomed to the format without burning you out. Don't obsess, don't feel guilty when you aren't studying (that way madness lies, especially if you carry that behavior over into law school), and do what works for you. How did you study for the SAT? Do that, but more intensely. How your brain works has not changed since then. It takes a year of law school to warp your mind.