I got A*A*A*A* in Maths, Chemistry, Physics and Biology. Honestly, I think a lot of this was down to natural ability. I have a good memory for facts and I was able to understand everything in my A-level courses without any problem at all.
This is especially the case with chemistry. I have numerous friends who found it very difficult to grasp things such as reaction mechanisms whilst I understood them almost immediately. For this reason, if you find you don't understand something, ask someone (a teacher or maybe even a friend) so you can figure it out as quickly as possible.
However, I also spent lots of time going through the textbooks of all my subjects and wrote out my own comprehensive revision notes. I recommend doing this as not only do they provide invaluable revision aids closer to exams, but the very act of writing everything out helps ground everything in your memory. This is pretty time consuming, but I enjoy making notes so it didn't bother me too much as it was pretty relaxing (you can watch TV or listen to music as you do so). I also did this instead of homework during free periods. Actually, I was fortunate in that I never really got much homework so I had more time to spend on my own notes.
Some textbooks are very waffly and contain too much information. If you find this to be the case, I recommend buying a CGP revision guide for your particular course and making notes from it. They're usually concise and to the point.
Try to make notes throughout the entire year. Don't leave them all to the last few months. Start making notes on day 1. If you feel up to it, you can even read ahead and try to teach yourself some of the course your teacher hasn't covered yet.
It was only much closer to the exams when I started properly revising. I didn't actually spend all that much time at it, because I was able to remember the courses very well. Instead, I spent the most time doing past papers. I strongly recommend doing as many past papers as you possibly can under timed conditions. I cannot emphasise enough how important doing past papers under exam conditions is. I did as many as I possibly could.
It's also good to leave one for the night before the exam so the general layout of the paper is fresh in your mind meaning you have a good idea of what to expect.
I didn't really have a strict timetable or routine. I just squeezed in note-making whenever I felt like it (just make sure you don't leave all the notes until the last minute) and did most of the past papers in the days right before my exam.
I hope I've been of help. Good luck with the coming two years. One last piece of advice I can give is: don't work all the time. Enjoy sixth form. Lower and Upper Sixth have been the best two years of my life so far.