A good start is to understand how authors write so you can figure out how to better read. Most authors use consistent "clue" words to tell you when they are stating their conclusion, and others to tell you when they are introducing an example. The key to good passage reading is to be able to unlock that code.

For instance, most of the time when an author says "however" he is introducing his actual point after having first described an opposing point. This can be confusing to readers and it is important to learn to look for the clue words to improve your reading skills. I teach my students to try to sum up each paragraph with either one key sentence or to try to sum it up with 5-6 word sentnece. Once you have summed up each paragraph it is easier to find the main idea.

But Reading comp is not all about the passage. be sure you are also paying close attention to the questions - you have to know what is being asked, be able to find the information in the passage and then be able to weed through all of the attractive wrong answers the GMAT has set out for you.

Best of Luck.