This is a really tough method to use - and, no, you can't use it on every single sentence or you'll never get through the test on time.
The general idea is that, when you run across a hugely complicated sentence, you try to focus in first on verbs / actions, and then the nouns performing those actions. That gives you a basic idea of what's going on, before you then try to add in all the extra info (modifiers). And maybe you even just skip some of the extra info in the end because it's too complicated.
If you're finding that you need to do this on many or most of the sentences in a paragraph, then that paragraph's too hard. Try to get the main idea of that paragraph (first or second sentence) and then don't worry about the rest - if you get a question about some detail in that paragraph, oh well. Guess and move on.
If you're finding that you need to do this for most of the passage, then this passage is too hard. Still go for the main ideas of each paragraph because that should help you to answer the main idea question, but then you're going to get 2-3 questions wrong here.
That kind of an experience also tells you, though, that you need to improve your reading comp skills in general, which involves spending time reading "GMAT-like" writing and getting used to these kinds of convoluted sentences.
You can get some ideas about how to do that in the below article:
http://www.manhattangmat.com/blog/index ... rehension/And, of course, keep asking questions here :)