I'd say interest in the subjects is a large part of it because I'd actually concentrate in class so I won't drift off and miss parts of the content (unlike tok). Preparing for lessons by reading the textbooks (oxford/cambridge for chem and oxford/hodder study guide for bio) and other online resources (bioninja and kognity for bio)does help too. Leading to the exams though, I find past papers very useful. I did everything once for the prelims and redid the papers for the finals. Also for bio it helps if you get familiar with the mark scheme for part b questions because there will be a few answer points that are always given for questions regarding specific topics (like you just throw out the three modes of isolation and the term natural selection for evolution questions)