Kaplans was fine for me...we went over it once in class and did like one for practice...and that was all i did to prepare for the writing sample...ever. And i ended up fine (50th percentile, nothing great but i dont think it matters anyway).
I don't think theres much to know for it except just to get used to the format (the three objectives), and know that you can use hypotheticals if you cant figure out a real example.
I dont think US med schools really care anyway, so unless your from Canada, dont use too much of your time to prepare for the writing sample.