1. Read the whole passage first, then answer the questions?
2. Read the questions first to get an idea of what to look for, then scan the passage to answer the questions?
3. Use a "scan as you go" approach to answer the questions, trusting that the first question relates to the first part of the passage, the second question relates to the second part, etc.?
The answer is: it depends. Students have different cognitive styles, which makes it impossible for me to recommend one specific approach. There's also the issue of time pressure: if you're pressed for time, then certain options, like reading through the entire long passage first, may not be available to you as the clock runs down. This will affect your strategy.
A lot of test prep companies recommend simply reading the passage first, be it long or short. There are obvious advantages to doing this. First, you now have the entire passage in your head, making it easier for you to scan more quickly for whatever you need to know. Second, because you've internalized the passage, you can figure out context-based relationships better. Third, you probably have a clear notion of what the passage's main idea is. All of this means that, if you've read well, you should be able to march relentlessly through the ensuing questions as easily as Einstein doing basic algebra.
But what if you're not the type who can get through a long reading passage without falling asleep? My first reaction to this question is: I feel sorry for you and your future college career. The fact of the matter is that you're going to be doing a lot of reading while in college, and much of it will involve literature that you just won't want to study. So toughen up! The only way around this obstacle is through it. My second reaction is a little more moderate: true, many passages will be boring, so find a method that works for you. If Method (1), above, doesn't work, then try Method (2). This strategy has also been recommended by test prep companies before. Especially if you're pressed for time, you need to read for information in a targeted way as opposed to trying to swallow the entire passage in a single mental gulp.
Method (3) has its uses, too. I find it especially helpful if I see questions about vocab-in-context: "In the context of the passage, the word mortal on line 36 most likely means..." That sort of question can be answered quickly, and without reference to the entire passage: just scan the lines above and below it.
Don't get trapped into thinking that there's only one magical method to beat the SAT. There isn't. In the end, the so-called "tricks" that test prep tutors teach their students are nothing more or less than good old reading skills-- the selfsame skills that you'll be using (and hopefully honing) while you're in college. Every method has its merits and demerits. Figure out what you're comfortable doing, make sure you can do it efficiently, within the allotted time, and be ready to switch to a Plan B if your preferred method doesn't seem to be helping you.
As Bruce Lee said: Be like the nature of water, my friend. Be flexible; shape your technique to fit both yourself and the situation. What that really means is: master several different techniques so that you don't run out of options when it's crunch time. True freedom comes from having options. Having options comes from self-discipline, and that involves time, effort, and focus.