For Chemistry, and I cant stress this enough, just do past papers. Even if you semi-understand a question, simply doing some past papers will helps you develop an understanding and also allow you to identify the key components of the question and what the examiner is looking for. Its important that you try to do hard question on topics that you struggle at, so they become strengths before you enter the actual exam.
For Biology, just try to stay on the SPEC and concentrate on getting a detailed understanding of the topics. In terms of exam structure, again practice does help, because you can see patterns in the kind of thing the examiner is looking for. For example, for OCR biology, if you answering a question related to planning a sampling investigation, you always get marks for saying: Use a key to identify the species, repeat the experiment in different locations and repeat the experiments over different seasons.