The MMPI is for the most part an "honesty test". I am not saying this, the proctor and the shrink himself told us. This test is designed to get your personality profile, sure, but also, to see how honest you are within yourself. Out of the 500+ questions, about 20 of them should take more than a few seconds, and that's where people get in trouble. They keep trying to outsmart the system, don't do it. Answer the questions at a "rapid' pace, and move on.
Will fly for food has great advice, and Airinc has an outstanding prep, specially for the cognitive test, I couldn't imagine taking that test without their prep.
Good luck.