Focus on structure not content
When reading LSAT Logical Reasoning passages, focus on the general structure of an argument, rather than on the specific content. This is obvious for questions that ask about structure, flaw, or for a parallel argument. But you should also keep this in mind for other question types as well, particularly when difficult content appears. Consider PrepTest 45, Section 1, Question 2. To do this question quickly and correctly, you can more or less skim over the bits about chlorofluorocarbons and the ozone layer; indeed, you do not even need to know what those things actually are in order to get the question correct. Notice that the author's premise provides a time reference to 300,000 years ago, yet the conclusion is about today. Time references should jump out at you when reading passages. The author makes a time comparison assumption that what was true in the past (300,000 years ago) is still true today. How would you weaken this argument? Find an answer choice that shows that a difference exists between the past and the present, which is what (C) provides. As another example, see LSAT PrepTest 50, Section 4, Question 25. The main point is all you need to extract, and thus you do not need to read the details of the premises -- simply skim over them to ensure that they are premises.
Be wary of any discussion of morals
You will commonly encounter passages that in some way or another bring up the word "moral" as an adjective or a noun. Do not attempt to search your soul, or try to get into a moral debate with the passage. Simply focus on the structure of the argument and answer the question. See LSAT PrepTest 50, Section 1, Questions 12 and 25, or LSAT PrepTest 50, Section 4, Question 4.
Recognize when an argument takes an extra step to provide advice or a prediction.
When making arguments, people often do so to provide advice to others or to predict future events. Some LSAT arguments do this, some don't. If an argument does, then make a note of it as this will help you identify structure and assumptions. Conclusions that provide advice often contain words such as "should" or "must" -- see LSAT PrepTest 48, Section 4, Question 10. Conclusions that predict the future often contain the word "will".
More importantly, recognize when an argument does not take an extra step. And, even more importantly do not attribute a conclusion to an author that s/he does not make. For example, look at the conclusion in LSAT PrepTest 48, Section 4, Question 9. The argument does not advise women to increase folate and vitamin B6, or to ensure that they get enough, or any advice at all for that matter. Also, the argument makes no prediction as to what will occur with this new knowledge. Incorrect choices will refer to conclusions that test-takers guess at, rather than the conclusion that is actually made in the passage. For more examples, see (E) on LSAT PrepTest 47, Section 1, Question 1; (A) on LSAT PrepTest 48, Section 4, Question 22; or the passage for LSAT PrepTest 50, Section 4, Question 20.
Distinguish between "logical" and "reasonable"
People often use the words “logical,” and “reasonable” as synonyms. Yet, the two describe different concepts, and, distinguishing between the two concepts will assist you on the LSAT.
Let's start with “logical”, which means that claims abide by the rules of logic. Consider a simple argument:
X is always followed by Y. X has occurred. Thus, Y will occur.
The above argument is logical. Notice that content is completely irrelevant as to whether the argument is logical or not. X and Y could be replaced by anything, and the argument would still be logical. Even if we place something absurd in for X and Y, such as this:
Smashing one's finger with a hammer (X) is always followed by one smiling (Y). John has smashed his finger with a hammer. Thus, John will smile.
This argument is also logical, despite the absurd content. However, this argument is not reasonable. The term "reasonable" means that, based on your knowledge, experience, and value system, statements make sense to you. What is reasonable to one person may not be reasonable to another. For instance, consider politics -- what one person considers a reasonable plan will be vehemently opposed by another.
When analyzing arguments on the LSAT, do not try to determine what is reasonable. Do not search your personal opinions or feelings. Rather, focus on the logical (or illogical) structure contained within arguments. As an example, look at PrepTest 49, Section 4, Question 16, and see the difference between choices (A) and (E).
Think, don't feel
Be wary of any content that you may hold strong opinions about because you may find your emotions rising in response to the statements made by an LSAT passage, and thus focus on content rather than structure. As an example, see LSAT PrepTest 48, Section 1, Question 18, which is directed at "You", the reader, not some third party. Also be wary of an argument that contains charged political terms. See LSAT PrepTest 50, Section 4, Question 4. The author states "as modern liberals tend to believe", but you can entirely ignore this phrase and still analyze the reasoning structure of the argument.