Putting in enough work, but not so much that you burn out.
Don't be the student who does the set homework on the bus to school on the day its due, but definitely don't be the student who works from 6AM until 1AM 7 days a week. Those students are both likely to get mediocre grades at best, while those in the middle achieved higher grades, but the one who did no work has a much more enjoyable year (or at least that's what seemed to happen grade wise), so overworking in my opinion is worse than not working hard.
In subjects where there is a lot of past papers available you should aim to complete at least 1 in every subject every other week, as soon as you finish the first unit, as this helps keep both the knowledge and exam style at the front of your mind as well as the current work.
If you get a cold or other illness, where you feel well enough to attend school, but not right, take a step back, as working too hard will make the illness last much longer, and 60% of normal effort for a few days, and then returning to 100%, is much better in the long run than 80-90% for up to a few weeks.
At exam time, when you have an exam the next day just focus on that unit, I would personally say the same if you have one in the morning and one in the afternoon (revise for the morning one the night before, and the afternoon one in the time between exams, as you should get at least 90 minutes), in subjects where some things from one unit may be relevant in another unit, but don't attract credit in that other unit, make sure that you are 100% over what is in each unit.
Read mark schemes before making revision notes, to give you an idea as to what the examiner is looking for, and try and keep on top of them as you learn the topic. Also try and make sure your revision notes are short enough, and have enough white space, to be read in 15 minutes just before your exam, and try to photocopy them if hand written so that you still have a copy of any you have to bin just before the start of the exam (reading notes outside the exam hall can both distract your mind (the 5 mins before an exam is always worse than when you are actually sitting it), and give you one last look at key points) .