GRE reading passages are deliberately drawn from fields you do not know — mineralogy, art history, a controversy in evolutionary biology. That is the point. Nobody in the room is an expert, so the exam can only test reading.
Read for structure, not content. On the first pass you are looking for four things and nothing else: what the author's position is, whose position is being reported and disagreed with, where the passage turns, and what each paragraph is doing. The details are still on screen — you can go back for them, and you will read them more purposefully when a question sends you.
Mark the turn. Almost every GRE passage has one: *but*, *however*, *nevertheless*, *more recently*, *what this account overlooks*. Everything before the turn is usually the view being set up; everything after is usually the author's. Finding the turn answers the main-idea question and half the tone questions before they are asked.
Separate the author from the people in the passage. A passage that spends two paragraphs explaining a theory is not endorsing it. The most common wrong answer on the whole section is a correct statement of somebody else's view offered as the author's.
Answer from the text and only the text. A GRE answer is defensible by a specific line. If you cannot point at the words, you inferred too far. The trap options are the ones that are reasonable, plausible and unsupported.
Select-in-passage and select-all-that-apply. Two item types that reward care. The select-in-passage item wants a single sentence — find the one that does the job described, not the one that says something true. The select-all item has no partial credit: all correct choices and nothing else, so treat each option as its own true-or-false question rather than picking a set.
Pace. A long passage is normally worth three or four questions and a short one is worth one or two. Time spent on the first read of a long passage is repaid; time spent on the first read of a short one is not. Read short passages once, closely, and go straight to the question.