I can help you with Physics and to an extent with Chemistry but not really with Biology; Physics to A2, Chem to AS and did Biology for the first 2 weeks of sixth form but hated the way it was taught at my school.
1) Tips for studying - there's no real generic tips that apply to everyone, it depends on how you learn. A lot of people that will have gotten the same results as me in the same subjects probably made perfect notes on every detail and them used them to revise but I didn't once look back on the notes I made in class. Text books were my bibles, they were the main resource I used when revising and really you can't go wrong with them. For Physics I found the best way to revise a topic was to start with understanding the concepts as best I could and then learn definitions and things like that after, eg, the Principle of Moments - I found this quite hard to understand at first and I would have just wasted my time if I had tried to learn the definition of it before I actually understood it properly. As I said, it's different from person to person so if your friends do things a completely different way, don't worry.
Studying times - (can't really remember for AS) but at A2 the closer it got to exams the more work I started to do. My personal preference was to start off in September lightly and not really do that much work (I always seemed to have forgotten what I'd learnt in September by the time it came to January exams so I figured I may as well take it easy at the start!) and then ease into doing an hour of work for 2 different modules each day. In January of this year I only had 4 modules so the workload wasn't that big (ooh that reminds me, if you work hard at AS and get good results then you won't need to re-sit anything in year 13, which is a god send come exam time!) and I aimed to start looking at past papers by mid-November and to have completely finished the module by mid-December (this is when we usually finished it in class, with a lesson or two spare before the Christmas holidays). To sum that up, you just need to find a rhythm that you're happy with - if you feel confident that you're making good progress then you'll feel more prepared come exam time.
Past papers - Not looking at past papers before you go into an exam is almost as suicidal as having never opened the text book for the exam you're sitting. They're the best tool you have and there's lots of them - use them!! Get familiar with the layout, the wording and check the mark scheme for exactly what they want you to put. Leave a few for you to try properly, refrain from checking your notes and pretend it's the actual exam, if you do badly it should shock you enough into revising harder! Don't be afraid to repeat the past paper to see if you can improve your score - sometimes they almost copy questions word for word from a past paper and put them in your exam.
2) Tips for school in general - the main point would be these are
your A-levels, so do what's right for you, even if that means ignoring advice from your school. Don't take what they say as 100% correct - you'd be surprised how many teachers don't know how things like A*s are awarded or how module combinations in Maths work.
3) Tips for Coursework - in the Sciences the only coursework we have is Assessed Practicals which are 20% of the A-level. ''Yippee!" you may think. Think again - these are not easy UMS. For whatever reason, the grade boundaries for these are devilishly high and pretty unforgiving, make sure you revise for them!
4) Mistakes I made - probably one of the biggest mistakes I made was the one I pointed out above. In Physics at AS I did really well in the first exam and was finding the second module quite easy so I didn't worry that much about the practical thinking it would be easy too and came out with an average mark in it, then came out with an average mark in the second exam and only just scraped an A overall, so be careful!
5) What I wish I'd done differently - apart from the above I don't really have many regrets, I thought about the way I worked at A-levels as trial and error, I tried different methods of revising until one worked for me and I stuck with it.
6) What do you need to do to get 90% - simple, you need to be motivated and passionate about the subjects you're studying and you need to work hard. If you do that you'll achieve whatever you want to. What did I do? I stayed motivated and kept thinking about how I'd feel on results day when I did really well which pushed me to keep working harder and more efficiently until I managed to achieve the grades I wanted.
7) Books you should read - I didn't read any and I'm a bad influence - read about things you're passionate about and you will come to be thankful you did when you go to uni interviews and you have to spend time talking about why you want to study the subject you've applied for.
8) Resources I used - things like
http://www.s-cool.co.uk/ and
http://www.khanacademy.org/ are great for all the Science's and
http://www.astarmathsandphysics.com/ is great for Physics.
Oh and if you enjoy your subjects it will make learning about them a whole lot easier!