I started seriously studying for it about 2 weeks before my O's. Before that I only study when exams are about to come... plus sometimes nothing to do take out read those topics I like. But don't be like me lah... my grades mostly sucks except for Chinese.
For modern history, I mostly is online research one but only on the topic of China (addicted to it)... exams come my classmates always say I sure to choose the essay on China to write one (of course lah... I know those topics better than Russia or Germany).
For Maths, my school started giving us exam papers from other schools and past year papers from my school to do after the June holidays... even if your maths no good also can become good... plus can discuss with friends so I learn more than what I would have done so if I do on my own... I don't like reading the Maths textbooks to find out how to do.
Chinese paper... I read the ci yu shou ce for a few days and still stuck on the same book... three days before the paper... I change to do those tian xie han zi exercises... in the end... no need to retake. But if you know me lah, you know that I seldom speak English... even if you speak to me in English, I still can reply in Mandarin (this one, don't learn from me, you will lose out if you only know Mandarin but not English) and I read 2 Chinese newspaper everyday. I am addicted to Chinese storybooks, especially swordfighting novels.
