I was getting about 60% on critical reasoning and I purchased the PowerScore Logic Reasoning Bible (for the LSAT, because I am studying for that too and it is similar to the critical reasoning). I went through the whole book and now I am scoring over 85% every time I do a logical reasoning or critical reasoning section. They also now have the Critical Reasoning Bible for the GMAT by the same author, I highly recommend it. I learned the different question types, how to identify parts of the arguments, incorrect questions to look out for, and solved problems dealing with every question type.