A logical argument includes the premises or key evidence, an often unspoken assumption made from the evidence, and a conclusion. For instance, say the question's premises are 1. law enforcers are guardians of society; 2. blacks are victims of society. Conclusion: a conflict of interest exists between blacks and law enforcement -- so argued Eldridge Cleaver. The unspoken assumption is that law enforcement is racially profiling blacks as criminals, which accounts for their victim status. Your task, based on this syllogism, is to determine whether or not the conclusion is true.
The key to unraveling an argument is to ask the "why" question the logic leads you to. Don't waste time attacking or defending the premises; the LSAT questions assume they are true. The central question is why the author would make the assumption. The assumption is made to prove a conflict of interest. Now, apply the "why" question to each premise. Why is there a conflict? Because law enforcers guard society. Repeat: Why is there a conflict? Because blacks are social victims. The logic of the conclusion is solid as far as the premises go, because each premise answers the "why" question.
The LSAT will ask you to support or refute the conclusion of the argument. To support, you can point not only to the premises offered but also refer to others not mentioned that will extend support. These unspoken premises are called assumptions; they are also very useful in refuting arguments, since you can point out implicit assumptions that undermine the conclusion. For example, supporting Cleaver's conclusion, you could add further instances of police violence against blacks; to refute it, you could offer other reasons why blacks are social victims, other than police involvement.
Whether you are asked to increase or lessen an argument's efficacy, the key is the assumption, which you should, through continual practice with logical questions, learn to recognize; you will always find sufficient evidence to support or refute based on the unspoken premise. Recognizing the hidden assumptions in arguments is key to mastering the LSAT Argument Section.