I think the best way to see quick improvements on your grades is to chat to your teachers in those subjects and ask to go over your work and past papers. together you can highlight the areas you struggle with most to target in your revision. If you work on those with tips from your teachers you'll see fast improvements.
Try to make the most of your revision. I would recommend to focus on the subjects you have that day in your revision, stay after or before in the library making notes / flashcards, then closer to the exams swap this out for memorising the flashcards, doing past papers and getting friends to hold the flashcards and quiz you.
Try sticking to just the work hours of the day e.g. 9-6 and always have a rest day once a week where you do no studying or work at all. It's important to rest your brain and prevent burnout otherwise the studying can become useless.
If you commute, use this time to catch up on homework or revising as this will make the most out of your day and should stick to the only studying 9-5 or whichever times you set.
For improving memorisation, try out the pomodoro technique as this allows you to use your brain better and maximise memorisation. Also, it may sound silly but visual mind maps also really help as it makes you use the right hand side of your brain not just your left, and visual information is much easier to memorise than long textbooks.
Hope this helps!
Vee (kingston rep)