I'm on my way to law school this fall and I will echo what someone said earlier - take as many practice LSATs as you possibly can - they are more beneficial than any commercial course.
I took the Powerscore course. I did the homework, understood what was going on in class, and took the 5 in-class tests they gave, and my score rose upwards of 10 points, but when I took the actual LSAT, I went back to the score I received on my diagnostic. Perhaps it was a fluke, but I really think it was that I was not as familiar with taking the actual test as I should have been. I went back to the practice tests, and took 20 of them between the two testing dates, and my score shot up 10 points.
The more times you take the test under timed conditions, the more familiar you are with the exam, and the more confident you will feel as you take the test, which may be the most important factor. While the course may have helped, I really think you need to take the time and pound out those practice tests - it may be a pain in the @$$, but if it gets you in where you want to go, it'll all be worth it. Especially if you do it before you take it the first time - believe me, no matter how much you improve, the average isn't going to be as high as you want, and with the weight placed on this one stupid test, it greatly affects your chances of getting in.