In the months before the exams of 'revising', or in other words sitting in front of the textbook, I tend to condense the whole textbook down into a set of notes. If the notes are then flashcard-able that also becomes an option.
Other than that I'd say I'm in the same boat as the person before, for most mocks so far it's been a revise on the night/morning before basis, but if there has been a prolonged period of revision, as there was for my actual GCSEs, then it's all about the past papers, they're the undisputed best way to do well in an exam. Unfortunately, I now do 4 essay subjects, which means doing past papers can take anywhere from an hour to two hours, or even three if I'm really putting myself in exam situations, and it's a lot harder to just rely on questions from the internet, so I'll probably be using flashcards a lot more.
Also if you're doing GCSEs and you don't have CGP guides what are you doing?