Just to add to Sajjad's post, Nightblade's RC/CR thread is great.
The basic and most thing about RC and CR is passage comprehension. And it's also true about the GMAT exam altogether.
So, you need to forget about how many questions you get right in a passage with 3 or 4 or 5 questions. Though analysing your performance is good, take one passage at a time - even during your actual/mock exam.
Read articles from Economist, NYT and WSJ for improving reading speed and overall comprehension.
Half the work is done if you get accustomed to the main idea of passage and general flow of the passage. Questioning why does author mention a certain object in a passage is good way to approach a passage.
Finally, check the pattern of your error. If there is one, then that's great as you have mastered some question types. If there is not, then it is problem as you need to change your approach altogether.
Take LSAT passage for practice only if you run out of GMAT Official ones.