You read for topic, scope, main idea, author's attitude, STRUCTURE!
Basically, you are making a roadmap for the passage. You know where to look when asked. When you have a fairly clear idea how the passage is structured, you know what point the author is trying to make and how he makes it. When you have established your systematic reading approach, you rarely need to go back to the passage to answer global questions such as main idea, main purpose, author's attitude and structure. You will find it is easier to answer inference questions (most difficult RC questions) because you have a whole picture. You will know where to look when you are asked about a specific detail.
It's impossible to memorize all the details and you will get lost trying to remember them.
Study the explaination of Kaplan coming with every test. That's the proven way to read. If you visit LSAT forums of US students, every test taker hit 99 percentile (>173) would tell you the same thing: structure and taking notes on the side. Like marking MP? on the side of any sentences that look like the main point; C for critic; A for author; Def for definition of a key term; 1,2,3, a,b,c for numeration of lists; double arrow for contrasting points...etc. You get the idea.
With practice, it will click. There is a mechanical way to attack LSAT passages. Follow the principle; develop your own marking system.
Of course, you need to solve any problems related to vocabulary and difficult sentences first.
One more tip I learned the hard way: always go back to the passage to check your answer for a detail question. NEVER answer it with your vague impression.
sigh 今天和研究生班上的小美们讨论了一下 惊喜地发现几乎每个人都考过lsat 他们说,阅读对他们很简单 和sat得很像~ 基本上他们都不用准备,直接都上去了 还说,我们外国人,要注意read between the lines 要读逻辑,记细节,判断框架,还要read between the lines 真不是人干的活儿