Always identify the reasoning error before viewing the answer choices.
If you are unable to spot the error in the argument, do not jump to the answer choices in the hopes of finding a choice. You may find a choice that sounds good to you (even though it is incorrect) and then find a way to justify it. You can find a way to justify an incorrect choice, much as you can find a way to justify eating ice cream as beneficial to your health (e.g., Ice cream provides needed nutrients such as calcium and protein, thus it must benefit one's health.) Nevertheless, incorrect choices do not correctly identify the error, and ice cream is not good for your health.
Use a mnemonic device
If you are looking for a specific flaw (instead of hoping that the error will jump out at you), then finding the reasoning problem will be much easier. Thus, if the error is not apparent to you upon reading the argument (which is often the case), then use the following memory device to check for common reasoning errors that appear on the LSAT. Remember the following: “SAD PLAN CCC”, which stands for:
Sufficient vs. Necessary
Ad Hominem
Definition
Part vs. Whole
Lack of Proof equals a Proof of Lack
Appeal to Authority or Popular Opinion
Numbers vs. Percentages
Causality
Comparison
Circular Reasoning
Go through each error and ask yourself whether or not it appears within the argument. Thus, you would begin by asking yourself whether the author has confused Sufficient and Necessary conditions – yes or no? If yes, then go to the answer choices. If no, then move on to Ad Hominem, and continue through the list until you have found a reasoning error that appears within the argument, or you have finished the list.
If you finish the list without spotting an error in the passage, then go to the choices, but only try to find incorrect choices. For example, if you are positive that the argument does not contain circular reasoning, yet an incorrect choice describes circular reasoning, then eliminate that choice. This will likely help you eliminate one or two choices, but will unlikely get you down to a single, correct choice. Alternatively, if you have tried this but find yourself justifying incorrect choices even though you consciously know not to, then you could simply tell yourself that you have come across an especially difficult question for you, and, thus pat yourself on the back for putting forth the effort, fill in any blank, and move on to the next question knowing that you have spent an appropriate amount of time attempting the question, and have thus not wasted time.
Read up on reasoning errors
The above list provides a number of common errors that appear on the LSAT, but others appear as well, and, if you have practiced reading and identifying reasoning errors in other sources, you will likely find error questions on the LSAT easier. Check out: Fallacy Files
Learn to translate answer choices.
Perhaps the most difficult part of error questions is figuring out what the answer choices actually state. Indeed, you will likely encounter an argument that contains a fairly obvious error, but the choices consist of dense sentences with difficult vocabulary -- see LSAT PrepTest 48, Section 4, Question 11. However, even though the choices contain such tough sentences, they nevertheless often describe common flaws, such as those listed above. By being able to recognize those common flaws within choices, you will increase your speed and accuracy. Try the following. First, when doing error questions, seek to categorize choices as one of the common errors; you won't always be able to, but oftentimes can. Second, after you have completed five full-length tests, go back through all those tests, locate the error questions, and only read the choices, seeking to identify common error types and working on understanding the difficult language presented. This may seem tedious and boring (and it is), but after doing nothing for an hour reading LSAT error choices your ability to read those choices will likely improve.
Be wary of answer choices that "sound good".
Incorrect choices on LSAT error questions often contain errors that are easily understood and "sound good", and because of this, you may find yourself justifying the choice. If you determine the error in advance, you'll be less tempted to choose these, and, will likely eliminate them very quickly. Be wary of, but do not always eliminate, the following:
Appeal to emotions -- it's unlikely that you'll see this as a correct choice, but may very well find it as an incorrect choice. See (A) on LSAT PrepTest 47, Section 1, Question 1.
Circular Reasoning -- This seems to appear more frequently as an incorrect choice than it does as a correct choice. When the error is, in fact, that of circular reasoning, the correct choice often states something to the effect of "assumes what it sets out to conclude", rather than using the actual phrase "circular reasoning". See (B) on LSAT PrepTest 49, Section 2, Question 23.
"Fails to define" or "fails to specify" -- These choices are particularly attractive because oftentimes the author has not defined a specific term or other information. However, does the reasoning depend on that information, or is it simply information that would be nice to know? See (A) on LSAT PrepTest 50, Section 4, Question 2, and (A) on LSAT PrepTest 50, Section 4, Question 7.
Distinguish between Sufficient vs. Necessary and Circular Reasoning.
You will often encounter answer choices that state that an author has confused sufficient and necessary conditions, or choices that refer to circular reasoning. You must know the exact difference.
Confusing Sufficient and Necessary conditions works this way:
If X, then Y. Thus, if Y, then X.
In everyday conversation, some people will mistakenly refer to this as "circular reasoning".
However, circular reasoning provides premise(s) and a conclusion that are simply paraphrases:
If X, then Y. Thus, if X, then Y.