What is the most important thing in writing my personal statement?

It's all in the name - it needs to be personal. Your writing will not catch anyone's eye if it's not conveying the kind of person you are, the passion you have for your subject, and the special qualities you'll be bringing to the table. Everybody is smart at Oxbridge - you'll need more than intelligence to shine. Don't be afraid to describe the joy that your subject brings you, the extracurriculars that show off your passion, the hopes and dreams you have about a career in your subject. Just make it yours!