I've been doing some reading on mastering the GMAT. Here are some tips I've compiled.
Master one type of sentence correction and then move on to the next. Know your strengths and weaknesses in the subject area and in test taking. Spend more time studying for weaknesses. Familiarize yourself with the test format. Eliminate answers that don't make sense and use a multiple choice plug-in to derive correct answers. That can make you a more effective guessers.