-Work hard from the BEGINNING. It's very tempting to think "oh, I have a whole year, it'll be fine." You'll be surprised just how quickly a year can go by. And I can guarentee you'll look back and think "ffs why was I doing xyz instead of revising earlier." Leaving things until last minute has always been my biggest problem, and take it from me, it's not worth the stress at the end of the year when you realise just how much you have to do in such little time
-I know it might sound weird, but don't get obsessed with UNDERSTANDING everything perfectly at first. Another thing I did wrong. You WILL understand it eventually by going through it over and over again, condensing your notes, doing papers, etc. If you focus on initially completely understanding it perfectly, you will slow down the progression of your revision. Even if you're not 100% clear on it, move on to the next spec point, etc. You will understand it when you keep coming back.
-PAST PAPERS PAST PAPERS PAST PAPERS. You'd be surprised just how strict the mark schemes are on terminology. You think you understand and are writing a perfect answer, only to read the mark scheme and see that you barely got any marks because of how they want you to word it. Especially for science subjects, you NEED to say what is on the mark scheme. DON'T rely on just your knowledge.
-If you're doing an essay-based subject, ESSAY PLANS ESSAY PLANS ESSAY PLANS. These are probably the most useful things you can spend your time doing. For English, e.g. plan your essays around THEMES. Make them detailed and separate them by paragraph - topic sentence, key points, key quotes and then closing sentence. This is basically your essay, but so much easier to practise and memorise for the exam.
-Don't beat yourself up. If you're tired, take a break - you won't retain any information if you're not focused. If you fail a past paper, go through it again with the mark scheme and annotate it - more often than not, it's small mistakes that can be easily rectified, like terminology, lack of detail, etc. Being harsh and angry at yourself will only make things worse. Be kind to yourself and tell yourself you CAN do better and that you WILL succeed because you can