Every time you finish a new topic, do as many practise questions in Chemistry as possible and you should be fine. Chemistry is probably the more challenging compared to Biology.
I got A in Biology, Chemistry, Physics (258 260 262 out of 300 respectively) after getting A's at GCSE Chem+Bio and A*at Physics.
It shouldn't be too hard but just make sure you are prepared to work your socks off - have a glance at what your next lessons topics are gonna be before you go into lessons; try and understand the concepts being explained by summarising them in succint notes. So when you go into the lesson you can not only clarify what you didn't understand beforehand, you can revise what you already have learnt.