When I have a short thing to write about myself, I generally start with a mind map. I get a big bit of paper and write "Personal statement" in a bubble in the middle of it. Then I'd branch off and draw new bubbles with 'concentration', 'research' and 'general learning in'. From there, write down everything you can think of about each of the topics - what you think you want to do, what you expect to happen, what you think your professors will expect from you, what problems you think you might have, how you might get around them, what you enjoy doing... you can brainstorm more questions, as many as you like.

Then go through and pick out what you think are the most important points from each bubble (probably 3-5 for each) and join them up. I don't know what you plan to research, but let's say it's something like mediaevil English literature. For the research paragraph, you might write something like "I plan to research Marlowe's use of simile in his works from (date to date) because the topic isn't covered in the literature and both Marlowe and simile have fascinated me for years. A potential problem is that a fire in our library last year burnt our entire literature section between L and Q, but I plan to use inter-library loan to acquire source texts."
Colin ( OxfordMathsTutoring.wsnw.com )