When you first encounter the passage, read only as much as you need to get the general idea of the passage, without letting it drag you too much into detail.
Para I: read most of this one, as the main idea and intro to the passage will usually be here. Paraphrase in your own words:
researchers thought that sun spots are connected to weather changes, but could not find evidence until 2 scientists found a statistical connection related to wind direction chages. (you don't need to understand the east to west and back stuff - just remember that it's here, and come back to it if a question asks about it).
Every paragraph after 1st, read the first sentence and paraphrase. Think about how it links to the other paragraphs before it. Glance at the next sentence, and get into that as well if you see a "however" or other change of direction from the first. Otherwise, no need to really get into the rest of the paragraph right now - the remainder of the paragraph is usually "more of the same".
Here:
para 2 On the basis (presumably of the above theory), scientists thought that the winter would be severe. The however in the next line indicates a change, so read that too: it turned out to be mild, but some other dudes think they know why the prediction failed - implies that the theory above may still be correct, it just failed here.
para 3 The two dude noted that El nino also has an effect on whether. No need to read more 0 this is the explanation as to why the prediction in para II failed - they didn't take el nino into account.
This should be enough to answer the main idea questions and general structure questions. For the specific questions such as this one,
1) paraphrase the question stem in your own words - figure out what you are being asked. In the case above "what can we learn about La-nina from the passage?"
2) look for a keyword in the passage, and read around it. In this case, breezing through the passage will find la nina in the third paragraph, so read the sentence about la nina. The la nina sentence says that la nina is far less common - but less common than what? the previous sentence described el-nino - a case where the water temperature is high. It continues to say that the opposite, (low water temperatures, or la nina) is less common, indicating that la nina is less common than el-nino.
3) Go and look for an answer choice that says that - in this case, E. Be aggressive in eliminating answer choices: we do not know that la nina coincides with high or low periods of solar actvity (A), or indeed any connection between la nina and solar activity at all - just that it's rarer than el nino.
