Critical Reasoning

Well I don't think by practicing more you can improve your Critical reasoning drastically.

Its more to do with the way you THINK.
You should infact go over the OG again ( I am assuming you are already through it once)
Pick up the ones which u got wrong. Try to get them right. Before checking for answers, push yourself to figure out the exact reason why you think a particular option is correct.

In CR, most important thing is conclusion. Read the stimulus very carefully and draw a conclusion - even before reading the question and answer choices. Don't be in a rush to read the answer choices. Spend most of the time in the stimulus and understand it CLEARLY. This is important because, if you don't grasp the stimulus, the answer choices will confuse you. Generally when it is a weaken question you will identify the loop holes while drawing the conclusion.

Once you have done with the stimulus, read the answer choices one by one(I will leave you the choice of whether to read the question first or stimulus. I prefer reading the stimulus first though). Eliminate an answer choice before you read the next. If you think it is the right answer then keep it. But you should not be going back and forth in the answer choices. When you are evaluating a particular answer choice, focus on the conclusion and try to weaken it. Questions that ask you to weaken a premise are rare in GMAT, AFAIK.

Conclusion is the KEY. Also, I don't think lot of practice will help in CR. It's your basic thinking style and you need to improve that from inside. Just get a hang of all question types and tricks involved with each type. You can practice to boost your confidence though.