The best way to improve on reading comprehension is to improve critical reading skills in general. That can be achieved by using targeted GMAT resources and also by reading non GMAT material, such as the New York Times, Wall Street Journal, Economist, and Scientific American. Another way to improve is to keep a good record of the types of text (business, social science, hard science) that you struggle with, as well as the question types that you perhaps miss consistently (inference, main idea, detail, etc). This way you know where you need practice!