As a preface to my prep story, let me say that I wasn't feeling real ambitious in my pursuit of an LSAT score. My general goal was to get a score that would put me in the hunt at some of the better state schools in the South/Southwest. I wasn't trying to get into a T14 school, just a school that performed well in its state with the eventual goal of making my way to Florida or Texas (no state income tax + lawyers salary = the good life in my book). What I did worked for me and I landed where I really wanted to be, but that doesn't mean that anyone else should do what I did. I'm a law student, not a role model.
I spent a couple of months slowly poking at LSAT prep stuff, but only probably about a month doing anything of note. I'd honestly be surprised if I spent more than 24 hours on test prep.
For actual preparation, I read a book, took some practice LSATs, and looked at the prep materials LSAC provides on their website. The book was from some third rate test prep company (I can't remember who it was by, I gave it away a while ago) and I got it at Borders for around $20. It was kind of helpful, but not really worth the time or money. My practice LSATs were just old (but the newest available) LSATs that I ordered from LSAC. After I took them, I went back and figured out (as best I could) why I got answers right or wrong, which was very helpful. The test prep materials from LSAC's website were pretty helpful. The best one was sort of a mini-test that had maybe 10 questions in each section followed by a breakdown of why each possible answer was right or wrong.
After I read the book, I took my first practice LSAT and scored a 153. I wasn't overly happy with it, but I realized it wasn't too bad of a score either and I could get into somewhere with it (I also have a really good GPA). After that, I read LSAC's prep materials, looked over my first test, and took my second test a few weeks later. I scored a 161 on that one, which I thought was really good.
About a week later, I took the actual LSAT in June of last year and scored a 156. What do you know, right in the middle of my two prep tests (though rounded down, sadly). I guess I got what I asked for because that score made me competitive at most places, but not a sure thing anywhere that I really wanted to go. By some grace of god, I ended up at Florida State, so things worked out for me.
In retrospect, I could have been A LOT more dedicated to the cause, and I think a prep course would have probably been nice for the analytical reasoning questions if nothing else (that's where I missed about half of the questions I missed), but like I said, what I did worked for me.