When I did my A Levels, I found that different approaches were needed for each subject. For example, for geology I spent time going through the specification, making sure I had revised everything I needed to, then completed lots of past papers to check how well I understood the content. However, for ICT, I simply revised the mark schemes, creating model answers for each topic and then learning them.
I found that the biggest difference between GCSE and A Level was the amount of content - at A Level each subject was a lot more complex and went into more detail than at GCSE, and as well as learning the core content, we also needed to be able to apply it to different situations.
My advice would be to take the mock exams seriously and try out a few different revision methods to see what works best, so that you already have an effective revision method sorted for your subjects before the real exams.
There's no point just reading through the book because it won't stay in your mind that way.
Past papers are really important however I found that flashcards were a huge lifesaver for me and making mind maps.
Use lots of colour too (eg highlighters etc) if you're a visual learner like me.
The most important thing to focus on all of them is that you actually understand the concepts. Believe me it is VERY easy to write something down from a powerpoint or out of a textbook in class without actually understanding what they're saying. Go over your work repeatedly to ensure you can comprehend the ideas clearly + if someone were to quiz you on those ideas (hint: you can make your own or search for other people's quizzes on TSR) would you be able to answer all questions proposed?
And with AS Levels you can't just start revising 1 week before exam, you gotta revise like..now