Specifically for languages (from someone that did 4 at GCSE), it's very much about practise. Go over your verb tables and practise conjugating verbs and using them to write sentences every day. Have fun with it and make up some nonsense ones too (eg for Italian I use to write weird stuff like "sono una mela" = "I am an apple". But hey, I never forget the word apple anymore.)
Specifically for sciences, Art, Music and especially Maths it's a case of constant practise. Notes on, say, trigonometry are good, but not if you're not used to actually utilising them. Anyone that's done past GCSE can tell you that practise it what gets you anywhere, so just set aside maybe half an hour each day to practise the areas you're not so confident on. Past papers as others have stated are a fantastic way to do this. For art subjects you just have to keep rehearsing your chosen mediums/pieces in run-up to the end.
For humanities subjects (English, History etc) the key is to learn the structure of an essay (ie History would be intro, point 1, counterpoint, point 2, counterpoint, conclude etc) and don't write entire practise essays, but rather work on your critical skills and how you'd answer just one paragraph/counterpoint. For English specifically, I recommend flashcards with quotes on them.
For ICT, it's generally just common sense I found but using any programs on your computer will help; I used Excel to practise the formulae in spreadsheets.
Can't really help with any other subjects, as that's all I did.