1.) Brainstorm: list all your achievements, anything that interests you about medicine, your motivations into medicine, your characteristics and qualities you and others see in you, relate your achievements or stories to the characteristics and qualities you've listed
2.) Draft, revision, draft, revise, revise, revise. Keep writing. Don't worry about length at first, or awkward sentences or cliches. Its ok to write cliches like "i want to help people" while drafting. This is what revision is for. You can go back and turn these weak or cliche statements into your own unique stories, qualities, and motivations. Just get out your ideas first, then develop them into specifics, then trim down. (edit: my first draft was atrocious, and the intro was a mess just because I was trying to be "captivating". I'm glad I'm a month removed from that, and that no one replied to me on the PS reader list.. because it was just embarrassing how poor it was compared to what I finished with. But that's drafting and revising.)
3.) Get several different people to look at it. If you know any writers or people with English backgrounds, see if they'll help you out with the early drafting. Are any of your friends in med school already or know anyone who has gone through the process already, whether they be student doctors, doctors, adcoms, professors, lay people etc??? Get their opinions on the stories you use, if it gets their attention or they can understand your motivations and the theme you're presenting in your personal statement. Go back to the English experts to polish up your grammar, proofread, etc.
4.) Give yourself time - at least a month. I'm a good writer (despite how I often write here) and it took me nearly a month, but I'm not including all the time I spent in my head thinking about it or reading up on writing a person statement or brainstorming. I probably went through five or six drafts. Don't be too harsh on yourself either. Writing about yourself is one of the hardest forms of writing, especially personal statements for applications/jobs. Sometimes you'll think its golden, other days you'll hate your writing. At least I did, or maybe that was just me struggling with the old artist in me from my past and I knew I could do better if I kept revising or trying out different methods to see what worked and what didn't. Also, if you look at other statements or samples scattered across the internet or books, they may be a good starting point to get a grasp on organization or what an effective statement looks like, but don't compare yourself to them. You have your own story and your unique way of presenting yourself. Avoid language or ideas that you found interesting in other statements. Its OK to look at other statements for guidance or inspiration, but afterward get all of that stuff out of your head and just write from your own heart.
Good luck!
oh, and since it's SDN:
http://www.studentdoctor.net/2007/06/before-you-write-your-personal-statement-read-this/