I have a child in Y9 who usually does v. well in exams, especially Maths, Sciences & languages. His revision methods are different for different subjects, but mostly involve testing himself and practising recall. He has only just started his revision for exams after half-term, but these methods have worked well for him so far:
Maths: he is doing a few practice questions each day and then marking them.
English literature: he is building a quote bank for each character in the text he is studying.
French listening & reading: he has been testing himself on vocab.
French speaking & writing: he gives himself a topic (e.g. opinions about TV shows & music) and then writes or speaks about it for a few minutes, trying to use certain tenses & grammatical constructs.
Latin: he tests his vocab and grammar every few days on the CSCP website.
Physics: he has learned some formulae and is planning to do some practice calculations.
Chemistry: he has made lists of properties and reactivity and how these are explained by atomic structure and is planning to do some practice questions
Biology: he has learned definitions of keywords and is planning to do some practice questions
Geography: he has created lists e.g. causes & effects/impacts and is planning to test himself on them then answer practice questions. This is his weakest subject, so there is room for improvement in his revision methods.
Computer Studies: this will be his first GCSE-style test in this subject, so he doesn't really know what he is doing. He understands the concepts and has made lists of advantages & disadvantages of different types of hardware, sorting algorithms etc.