Making notes, making sure I covered the whole specification, writing practise essays (or at least planning them roughly), learning key points to include in essays. I did loads of practise questions for English language because I wasn't very good at it. Annoy your teachers by making them mark stuff. Plan your coursework at home before going into school and writing them in exam conditions (actually you might have already done coursework idk). I don't actually remember doing many past papers in my own time...might be because our school made us do them. Sorry that's a bit vague but my revision depended on the subject. (I got 10A* 1A, currently in year 13. GCSEs seem like ages ago!). Best of luck!