If you've been preparing for the LSAT, you're probably familiar with at least some of the flaws featured in questions from the Logical Reasoning sections of the test. Many of these flaws are exemplified in arguments against preparation:
This is a good example of an argument that lacks relevant evidence for the conclusion. The LSAT is a test unlike those encountered in high school or college. Having achieved good grades will certainly help your applications, but many honor students are surprised when they take their first practice test and find significant opportunities for improvement.
"I have a friend who did really well on the test without any preparation whatsoever. That sounds like a pretty good plan..."
First of all, this is a questionable notion; a lot of students who do well on the LSAT claim not to have prepared, and not all of those claims are true. With that said, this speaker appears to rely on the existance of an exceptional case; whatever this person's friend may or may not have done to prepare, people who take the test cold and do well anyway are very, very rare, and most of them are still unlikely to maximize their scores without any preparation.
"I'd like to prepare, but I don't have a whole lot of time or $1000 to spare."
This sentiment reflects a false dilemma. Voltaire warned against making the perfect the enemy of the good: If you don't have the time or funds to invest in a full length course, that does not somehow preclude all preparation! You might also consider taking a weekend course or a self-study approach (as long as you have good materials to work with).
"They say that test prep won't significantly impact my score--a lot of people say that the LSAT is like an IQ test, right?"
This is a common misunderstanding, and represents a flawed appeal to popular opinion. "They" are not a reliable source, and this is a flawed argument. In truth, the LSAT tests a specific set of skills that can be developed with practice and the right approach.
"My friend took one of those classes, and it didn't do him any good, so I'm going to avoid those classes."
The reference to "those classes" reflects an error of composition; the friend may have taken an unhelpful class, but that certainly doesn't mean that all preparation classes are unhelpful! This is also a false analogy; in such a context, a friend's experiences are not likely to be a very good predictor of your own results.
You may already be familiar with some of these logical flaws; make sure to avoid them when you determine your own plan for LSAT preparation.
Image: "Punctuation Marks Made of Puzzle Pieces" courtesy of Horla Varlan