I too went from HM to winner. Here are things I did that might have helped:
-- Improved personal statement structure: I went from a standard essay to an essay with headers for Academics, Research, and BI, and also Intro and Conclusion. I believe this is a requirement now, but in any case, I'm not a great writer and this drastically improved the structure of my essays.
-- Discuss things in more depth: I have 2 particular examples I drastically improved in my essays. For academics, I emphasized the number of grad classes I took as an undergrad (>10 quarters), and went into details on my 2 favorite courses (which motivated my graduate studies). For BI volunteer work, I discussed science competitions, both how they made me who I am today, and how my volunteer work (both interactive judging and just test writing) will hopefully impact competitors in the same way.
-- Read as many reviewer comments as you can: Many people close to winning only get good comments rather than constructive criticism. Don't just look for criticisms. Read as many good comments from friends' applications as possible and tweak your essays to support them all. You'll find ways to appeal to many possible reviewers.
-- Actually rewrite the essays: In theory, an HM candidate could resubmit their essays and win the next year. I know a few people who have just resubmitted and went from HM to nothing. You can definitely keep parts of your essays if they work, but make every effort to improve them. My personal statement was about half old, half new, and the proposal was entirely new.