1) Don't waste year 12. Not everything can be fixed if you do badly as you get bad predicted grades, AS really does matter.
2) Frees are not free. By all means have a few that you use to relax, but by doing work in some you'll be so much more relaxed.
3) Start revision around January/February and by Easter at the latest.
4) Don't be afraid to ask teachers for help, I was most of the time and it was a huge mistake.
5) Do loads of past papers and (if doing essay subjects) essays to hand in to your teacher (or mark yourself) to get a feel for papers and mark schemes. My Sociology teacher absolutely loved when I gave him extra essays to mark (bit strange haha) but it certainly helps.
6) Read examiners reports! They are basically free, written materials by the people who are likely to be marking your papers telling you what they want to see/hear from you.
7) Print off specifications and mark them as you go along showing what you've covered.
8) Do your homework. It's there to help you learn and aid you, so always make the effort to do it (unless there are genuine reasons you can't).
9) Start making revision materials early. Some of my favourites are notes, pre-made packs (thanks teachers!) and flash cards. I hated flash cards at first but they are so so helpful, but time consuming to make so start early. Other examples are videos, mind maps and more. (P.S. panic maps can be useful if you like mind maps, they're a thing my sociology teacher told us about where you write the main topic, surrounded by sub-topics and then key info for each, so that when you panic in the exam you can remember the sheet and hopefully have something useful on there).
10) For essay based subjects I would honestly say, learn the content, but practice practice practice your exam technique too!
11) Folders. At our school we have to have on big "every day" folder that we keep about 3/4 weeks worth of work in and then subject folders at home we put subject work in. Really helps with organisation.
12) Enjoy it. The year goes so much faster than any other academic year I have experienced (and I don't doubt A2 will be even faster) and you will probably meet some amazing people, so whilst putting in your all, be sure to chat to friends and enjoy yourself.
A-Levels are so much harder than GCSE but you don't appreciate being told that until you're deep in them, so work hard from the word go and you will do well! Don't be afraid to speak to teachers if you're struggling, they are there to help you learn and cope with the pressures so do go see them for anything you feel necessary. Also, try to get along with your teachers (I absolutely love 3 of mine so much but 1 of them hated a group of us for not being from her school that was specialist in maths (we were a sister school for humanities) and I ended up skipping so many lessons because she made them torture and I ended up hating the subject) and if you do have any problems with teachers, report them to someone higher in authority, though I doubt you will!
Agree with
@emiloujess on all of them if I'm honest, think they nailed it really..
If I think of any more I'll add them