Growing up I was relatively smart but never applied myself, so I was always sitting at around B's or 6's until I started my GCSEs in Y9. From then I decided to start actually applying my abilities and make an active effort to achieve highly; I managed to get all 7-9 at GCSE, and am now predicted A*A*A* going into my A levels next month. My advice would be to start as early as possible, keep solidifying your knowledge, read up on topics before you're taught them and ensure you apply your knowledge (practice tests/essays) as much as possible. However, as you're not long off, I would recommend (especially for essay subjects) looking at the people in your class that get the grades you want (consistently) and ask to look at their work. When you're doing your practice tests or papers, try to analyse why those people got the grades they did, and emulate their techniques in your answers.
The most important thing I can say is to have a 'grade 5 knowledge' of all your subjects - ensure you know the essential parts of each subject/topic, and then specialise in a few topics so if they come up you can grab some easy marks, if not, you'll have a good enough understanding of them to get a decent mark. For higher grades (7-9) you would need to bump you 'blanket knowledge' up much further which I'm not sure you'll have the time to do.
Ensure you apply all the knowledge you're gaining to keep it in your head, take breaks when you start to slow down, get enough sleep and stay well fed. Revision isn't the only factor in ensuring you perform well, keeping yourself well-maintained can go a really long way. If you're motivated enough though, you should achieve those grades once you find a revision method that works and consistently produces your desired results!