Stage 6 English (including English Advanced) constitutes a significant step forward from Stage 5 English. Whether it is through the more complex texts and modules that you will learn, techniques, or the more sophisticated writing, the standard expected in English Advanced exceeds that of year 10 English. In ensuring favourable performance, you should familiarise yourself with both the modules as well as the texts that you will be studying. This applies to both year 11 and (more importantly) year 12 and forms the fundamental element that will assist you in attaining and maintaining your desired standard.
Your knowledge and understanding of the modules and texts will subsequently support your writing ability. There is an emphasis on your ability to develop good quality responses, both short (to short answer questions, primarily those targeting unseen material) and long (to questions requiring long responses, primarily essays). The ability to effectively interpret and apply literary and visual techniques is also important given that they usually are an important part of your analysis. Based on this, it is important not to overlook the quality of your writing, and to seek feedback regarding your writing at every available opportunity, so that you can then implement any suggestions that you may receive and ultimately ensure that you maximise the quality of your responses.
Similarly, Mathematics Advanced also raises the standard by introducing more difficult concepts than those covered in year 10. The above post correctly identifies the need to regularly complete practice material. The reason for this is that while having a good understanding of each topic is essential, you still need to ensure that you are able to apply your understanding to the various types of questions that you may be exposed to, which calls for the need to complete as much practice material (including textbook questions and past exam papers) as possible on a regular basis so that you familiarise yourself with different patterns and methods required to solve different questions to a large extent.
Additionally, the more you attempt practice questions, the more likely you are to identify areas of improvement, which you can then work on addressing. As you do this, it is important to seek clarification about any concepts that you may not fully understand as soon as possible and avoid dismissing a potential problem, regardless of how insignificant it may seem.
I hope this helps!