1. Lay out a revision plan. Probably give yourself 2 months before each exam, and varying amounts of time on each subject (e.g. 2 hours in total on your best subject, 10 hours on your worst) 2. Do it. No distractions. No computer (you'll only go on TSR). Just get the textbook (or revision guide) and make notes on each topic. Then learn those notes (e.g. by writing them out, rewriting them smaller, etc). 3. Make sure you do some past papers.. 4. For some (AQA) subjects, it can be particularly beneficial to go on their website, take the past papers + mark schemes and make a 2 column table: in the left column, you have things like "ox bow lake", "4 processes of erosion", etc (for geography)... and in the right column you have a full and perfect explanation as explained from the actual mark scheme. Do this for all the previous years' questions, learn this table, and in the exam you'll come up against questions you've already learnt 5. Revise lots.
I did indeed have to work hard for my A-Levels. What I did though was do loads of past papers, read through mark schemes, and send them to my (very kind) teacher for marking. I asked for ways in which I could improve but basically worked by a$$ off. In mocks I would get Ds and Cs...in the real exam I got full UMS.