It doesn't really work to skim entire passages or read just the first and last sentence of passages.
However, what you can do is decide what parts of passages to read carefully and what parts to read without fully processing them.
Main ideas, key points, conclusions, etc. you should read for complete understanding.
Minor details, lists of facts, and other supporting information you can quickly read without fully processing them but noting where they lie in the passage.
For example, if the passage compares three marketing methods in a few sentences in one paragraph, you may be able to just note that the passage is mentioning three methods and be ready to go back to that section if you need those details to answer questions.
That way, when you've read a passage, you understand what it's about but have not gotten bogged down in the details and taken a long time to read the passage.
For some more RC tips, see this post.
GMAT Reading Comprehension Tips: Top 8 DOs and DON’Ts