Unlike math and grammar, I believe RC and CR depends more heavily on ones accumulated skills over a long period. One gets better with these things naturally when you use the language more often. Read different materials, write out your own opinions, participate in forum debate, all of these will help you getting the sense of the language.
However, most of us are preparing to take the test within several month, so long term strategy may not work. To get immediate short term improvement, I would suggest you focus your reading and practise on GMAT materials. (In other words, don't go read a novel or magazine etc.) I personally believe that the passages that were used in previous GMAT tests, such as those included in the
OG, would be what I'd be focusing on.
I still remember a long long time ago, when I was in high school, my teacher asked me to read a part of a fiction to my class. I didn't like descriptions of environments at the time. When I read a book I would usually skip all those about how sun was coming up and trees were green and stuff, and jump to the interactions among people. I read the passages word by word, and my heart went into it when I read it. I enjoyed reading it, and the story made my classmates laugh at several occasions. When I finished it, my teacher said that I didn't read the first paragraph well where the author described smokes rising from a river and such. That was when I first realized the difference between reading a passage word by word and reading a passage using my heart.
The reason I went to such length to talk about reading is that I truly believe that it is very important. In a test it is very likely that you'd read the words, without taking them in. Then you'd go look at the questions, and try to come back to find your answer. But your time is tight, so you don't have time to re-read the passage, therefore you have to find something that sounds close, and choose it. Often, it would be the wrong answer.
What you should do, is to really read the passage, using your heart. Practise this, you'll know what I mean. You'll feel the difference yourself. When you read it, you'll truly understand what the author is saying. When you look at the questions, you can almost answer it already, without having to go back to the passage. You only need to go back to it to get a confirmation, or get the details that you omitted when you read it.
Critical reasoning is the same thing. You must truly understand what the author is arguing. Then you'll see his logic, and be able to find his assumptions and missing links, and what would weaken and strengthen his arguments. I'm not a native speaker so I'm not very good at grammar, but I feel that trying to understand the sentence is as important as learning all the grammartical rules and idomatic usages in helping me to find the correct answer.
Sorry that I have rambled so long. Hope this could be of a little help to you.