I thought about that when I was applying last year but decided to do it myself with help from the writing center at school. And, it turned out fine.
I think the most important thing, IMO, is to have a compelling personal story that led you to medicine. Also, your commitment to community (in forms of community services or just volunteerism), leadership experiences, and academic and other notable achievement should be included. Plus, short mention of any reasons that contributed to weak points of your application(low GPA during one semester or low MCAT score) AMCAS's limit on the PS was 5300 characters(I had 5257 which turned out to be 916 words) AACOMAS's limit was slightly smaller.
Oh, and also, my pre-health advisor was helpful in giving me direction on my PS. And lastly, spend as much time as possible on revising.
Good Luck!