I'm a nontrad with twelve years between an undergrad CompLit degree and starting school this fall, so my path has taken some twists, as well. I'm also a professional journalist and editor -- or will be for the next three months (goodbye, age-appropriate point in my career path). So for what it's worth, here are a few ideas I found helpful when putting together my PS.
I didn't worry about the word count when I wrote my first draft. I concentrated on telling my story, as honestly and concretely as possible. In my case, my narrative wound up being more or less chronological, but I don't think it has to be. I also tried not to think about what an adcom member might think about this or that portion of it (do I look diligent enough? Am I interesting enough?). In fact, I told myself I wasn't even writing a PS -- I was pre-writing, to clarify for myself what this pull toward medicine was about for me. When I was done, I had something far more personal than what made its way into my final draft, but I had something to work with. Incidentally, this exercise also helped with my interviewing process.
I think it's also worth remembering that adcom members are humans (really!), and that as such, they like a good story. That's not to say that every PS has to be a narrative -- far from it, obviously. But writing that grabs people often provides imagery. For example, if you are juggling work and school, you might try to paint a picture of what that actually looks or feels like by including a few solid details. You could say, "I worked full-time while taking my science prerequisites, which demonstrates my dedication to this career path," or you could say, "Every weekday for the past three years, I have left my job as a widget-maker at 5 p.m., schlepped across town to Mudville U. to a three-hour lecture, driven home, kissed the hubby and settled in at the kitchen table for a long night of problem sets." Okay, so maybe I haven't convinced you with that example, but it's the end of the day, and I'm out of on-the-fly good ideas. Trust me -- concrete examples sell, and while they won't make up for bad organization or hazy thinking, they enliven your writing. And that ups your chances of getting your PS read by some tired adcom member.
I also think -- as others have said -- that getting others to read your PS is critical. I had at least two people whose judgment I trusted read mine, and made lots of their edits. It's easy to get attached to some kooky phrase or idea, and a trusted friend will convince you to axe it. For example, I had some long, confusing analogy involving ... what was it? ... oh right, a criminal lineup (don't ask!) that did not appear in my final PS, thanks to sane advice from others. Maybe you don't share this problem. Probably most normal people don't, come to think of it. But it still doesn't hurt to get other eyes on it.
I've really rambled on here. Enough already!
