The GMAT is an aptitude test---throughout it is testing a logic system: be very precise, assume nothing, and do not rationalize--the questions ask what must be true, not what could be true. This is most true concerning CR, as there are no math skills or grammar rules in the mix. The mistake that many make is to inject their opinions based on reality and/or evaluate the general topic. CR does not take place in reality---I always say that it bears the same relation to reality as the game Monopoly does. Similarly then, don't make decisions based on real life. Furthermore, you are evaluating the exact words, not the general topic. For instance, in a weaken question, you should ask yourself, "Do the exact WORDS of this choice make the exact WORDS of the conclusion less likely?" For most of the question types, the most important thing to know is the exact words of the conclusion---if you don't know that, it is very hard to answer correctly. In inference questions, in which there is no conclusion, ask yourself, "The words in which choice MUST follow from some words in the stimulus?" Finally, in CR especially, eliminate the four that are wrong and take the remaining one, as the errors in a choice are often more tangible.
I hope that this was clear and helpful.
