I know what you mean-i get writer's block too. Fortunately, I was forced to write mine a couple of weeks ago because one of my attendings wanted to read it before he wrote me a letter of recommendation.
First know that all PS must address 4 or 5 key issues; why/how you chose the specialty, why you would be a valuable asset to the profession, what type of residency program you are interested in, what type of practice you see yourself doing in the future (there might be one or two more...but basically they include stuff like the above).
Then decide what style you want to use; chronological, narrative, etc...
Then just brainstorm a bunch of crap for each topic within the PS. I just wrote a bunch of stuff down. Then weed out the stuff that doesn't seem as good, keep the best, and write your first draft.
I ended up going with the story style; started with a catchy paragraph about an experience I had which led me to choose my specialty and then added the rest in.
Look at examples of the different styles of PS, and don't forget to have at least three separate people read it before you submit.
good luck