I went to the olympiads (all the way through to IPhO), and my suggestion would be to know mechanics, but also to be able to do a lot of problem solving with the information given. The exams are challenging, but not impossible. In my year, I answered every question at least in part, and I'm pretty sure I got all or most of the multiple choice, and a good chunk of the extended. I know that I qualified very high up in the group of people who were accepted to the summer school, and in my year, I'd estimate that I got anywhere between 60 and 90%. Maybe it was lower; I have no clue. I know that a 40 or 50% is usually enough in chemistry, but chemistry has a lot more rote learning necessary and a lot of students don't do that in year 11.
It's also helpful if you can differentiate and know some basic differential equations and optimization.
[Also, I can't believe I'm actually posting here given how long ago I graduated... guess this is what happens when you have a month of free time in a foreign country with almost no peers in the same city...]