Hey for these subjects it often requires a lot of natural ability and being able to understand complex things which helps if you are naturally good at these subjects especially physics. However, do not let that deter you, because you may not start off at the same understanding as others but you can develop this by doing many questions based on the area from HSC and other exams and over time you will become better. That is one of the important areas to do past papers for these subjects and get them marked by your teachers, tutor or even other classmates who are achieving high marks ask them for some feedback. However, in saying that, I myself did do a lot of questions from different places, but not many under exam type conditions of 3 hour exams, but
I would rather do lots of questions based on areas I was having problems in or did not have much experience doing. I think this is important to do targeted improvement, because you may be consistently receiving 23/25 in a topic while you are receiving 15/25 in another area, by doing questions from your weaker areas you are more likely to increase your mark as you have 10 marks to gain opposed to 2 marks in the other topic.
Also,
exam technique is very important as there are things that band 6 students do in these physics and chemistry examinations that distinguish them from lower bands. For instance in Chemistry writing chemical equations wherever you can for questions even if they don't ask for it for example in a question that asks about Arrhenius acid and base theories and is 3 marks, providing equations shows an example and also your deeper understanding in being able to apply his concepts. So remember to write equations wherever you can. Also, when asked about processes such as addition polymerisation
draw diagrams representing the steps: intiation, propogation and termination, these diagrams add to your written description. These diagrams are also useful when describing properties of polymers as you draw the structure of ldpe and hdpe showing their relative amount of chain branching. In physics it is also helpful to
draw diagrams because if your English abilities to express yourself are not strong, a well labelled diagram can add to your answer and get you the full marks and using formula within your answer such as in rocket launches with momentum shows your knowledge of the topic.
Also, in these subjects the way you answer questions is very important. What I have developed over the year especially in chemistry, is an ability to predict the
marking criteria in exams. You have to understand that the markers will be following a strict marking guideline and this generally means you have to say certain specific things for each of the 3 marks, so being able to
break down questions like this is helpful in answering them. Although, many top students do write over the lines like myself, you have to understand that this information is normally adding value to their answer and is not irrelevant to the question, so make sure you are always answering the question. For example if there is a 2 marker about explaining properties of ozone and oxygen in terms of bonding and intermolecular forces for reactivity and boiling point. Do not waste time writing the definition of an allotrope. The way I would answer these questions is not done by many in formatting answers. What I recommend you do is writing a heading reactivity, underline it and begin writing, then write boiling point and underline this and continue writing, also include the structures of these molecules. This structure of breaking down your answer into headings is really useful in longer response questions, so please use this as it really makes it easier for the marker to see each section of their marking criteria answered so they can just tick, tick, tick instead of trying to see where you have written information if you write one massive paragraph. I have attached some examples below of me doing this in exams, it looks like I've written a lot but it's not that much since my writing is big, but note how I answer the questions in a clear, well formatted manner with only relevant information.
http://postimg.org/gallery/2yig9a1km/
Lastly help your peers and share your resources with them, because if it was not for this between me and my classmates, I would not have been able to improve my understanding as much and also it improves your HSC marks as if you're not first they affect your marks, but still for example I am first in both subjects at my school, but I have shared all my notes with my classmates and this has undoubtedly improved their marks as well.
Hope this helps