General preparation for half yearlies (based on what i did - i can neither confirm nor deny if this is effective and efficient studying):
Maths:
Asking previous year 12 students and looking back on Prelim and first term of Year 12 tests to see how the exams are usually structured. Some schools opt to put in previous HSC questions into their own internal examinations to prepare the students, but personally speaking, my school did not do this so it is best to find out if your school does. When taking the maths tests, during the 5 minutes reading time, i licked my finger and did as much of the MC questions as possible, marking the answer with a bit of saliva (disgusting.. i know) so you dont have to memorise the answer.
I was actually recommended by many maths teachers and tutors to utilise the reading time by completing as much as you can because maths is not one of those subjects where 'if you look at the questions early, some idea may pop into your head on how to solve them'. One of my problems during Mathematics x1 and x2 tests was time management. Rule of thumb: if it takes you more than 1.5-2 minutes to figure out a question thats a standard low marker ie. 1-2 marks - skip it and dont waste your time. Also, leave proofs and inductions for later on when you know you have achieved the maximum marks from the shorter questions.
English:
The number of times teachers have said to me NOT TO PRE-PREPARE AN ESSAY IS TOO DAMN HIGH! Regardless, i made up generic essays with strong body paragraphs and analysis before the exams, and simply adapted the memorised essay by writing a fresh thesis and closing statements on the spot. I have always had a lot of trouble with English and i knew the odds to achieving a good mark by going into the exam room unprepared was against me so i secured the guaranteed marks by making sure that i can at least get into the B to C+ range with my body paragraphs alone.
I also used this same strategy for the external HSC and the trials (memorising 5000+ words, word for word). Also, find a way to remind yourself to write as big as possible and have as much spacing between each word as you can (without there being consequences). You should try to write 9+ pages per essay. Even though they say its "quality not quantity" etc etc, ive always thought this was a bit of baloney and something that men with tiny yoohoos would say. I applied nail paint on one of my fingers to remind me to write big, everytime i started to forget.
Physics:
Really just gotta know your content off by heart. Always start any question with the definition of all the terms mentioned in said question, and follow it up with a clear and sequential lead up of how you get to your answer. Also, use diagrams whenever necessary, and always label everything on your diagram or it will be ignored when marked. Also put in a title for your diagram. If you are running out of time, you can also utilise tables or table formats to answer questions; as well as using dot points. However, make sure to include all relevant information when writing in point-form. I would say that these techniques apply to biology too, but not so sure about chemistry.
Managing stress is something that differs for everyone. I found that going jogging for an hour every weekend was a way for me to gather my thoughts and relieve pressures from school. I also enjoyed making to do lists and crossing them out - not sure why but i found that incredibly satisfying and stress-relieving. I also had this routine (somewhat religious by the ways i devoted myself to it...) of eating 2 banana's + a few bars of chocolate before going into exams - maybe its a placebo effect, but ive always felt that it helped settle my nerves and how it can replenish energy+electrolytes needed for recalling content during examinations. But most importantly, you must know your content from top to bottom. There really is no other way of better preparing AND reducing stress than that.
Hope this helped and best of luck!