To get only a P for the MCAT, all you really need to do is follow the directions and not make any blatant grammatical errors. If you want to do any better than that, I would suggest you practice with some of the sample questions available on the emcat site. What I also did was read the wikipedia articles on law, history, art, literature, the russian revolution, various painters, etc and studied the aamc examples of good essays that they have on their website.
Be sure to spend 5 min planning your essay before you start and leave 3-5min at the end to proofread. It's hard at first, but the ideas and the writing come much faster after a few practices. With all of that, I managed to hit an R