Hmm, the above advice is probably enough as one thing that can affect you in a bad way is worrying too much.
All I'm gonna say is DO NOT underestimate the essays; if it hadn't been for them I wouldn't have got in.
Keep in mind that maths isn't everything on the exam, and you shouldn't solely count on it.
The essays are tough under the time constraints, but can be especially rewarding if you write a decent one.
As an aside, may I suggest you apply for other schools as well (such as private school scholarships, if you already haven't). Many people do the MHS entry exam and ultimately realise (for some reason or another) they won't be able to attend. It's also good to have a back-up plan for anything, I guess.
Otherwise, good luck!