I have heard some people here discussing about creating and tracking questions failed. For those that do, how much time do you spending reviewing those question?
Until I really understood what I did wrong! For me, reviewing my mistakes made all the difference. It's very easy to look at a question you got wrong, and say, oh, well that was just a silly mistake, I don't have to look at that again. It is VITAL that you DO look at that question, and treat it with the same caution and respect you would show to a question that you got wrong because you didn't grasp the concept. Remember that the GMAT doesn't give a crap about why you got something wrong - a wrong answer is a wrong answer is a wrong answer, no matter how you arrived at your solution. Chances are very good that if you don't, you will repeat your silly mistake, or one like it, on the actual exam.
...there ain't no such thing as a free lunch...