This is what I did all:
Since in most of the subjects I was the highest achieving student in the class, I was often used as a 'teacher'. I helped a lot of students when they had problems. Believe it or not, but this is a great way of improving your knowledge of a subject. Helping out students, who are struggling, will not only help them, but also you. This worked great for me, because in Maths for instance, I sometimes had forgotten previous chapters, but by helping others, I managed to revise the previous chapters again and again.
Also do a lot of past papers (as you said), but only do them once you have finished the syllabus (book). Go through questions that you found hard in the past papers and keep re-doing them until you get full marks. (But don't just do it again and again, but leave gaps, as in don't do the same paper again and again, but do others in between.)
Try to finish your course as soon as possible. Try to learn the whole course with at least a month to go to the exams (I personally prefer 2 months). In those 1/2 months, do the past papers, do questions. Go to the library and find question books (you should be able to find books full of questions only) related to your course. Make sure you go through as many questions as possible. If you don't understand certain questions, ask your teacher.
Make sure you know EVERY question of past papers (no point remembering answers or something, but) make sure you know HOW to do each kind of question. I don't know what course you do, but in Maths, Physics, Chemistry, and Biology this is very helpful. Keep revising until you are 100% sure you KNOW it ALL!
Divide your time well. Don't do too much work at once. Have breaks and also create a time table. (If you send me your email address, I can send you a template of my revision time table in Excel.) Make sure you don't spend entire nights studying, because that's just pointless. People say don't do last minute revision, but if you haven't been doing any so far, I suggest you actually SHOULD do some last minute revision, especially if you are unsure about some topics (shouldn't be the case in the first place though). For me for instance it is extremely helpful.
If there are any formulas, etc to be remembered, make sure you learn them. If you just can't, just try to memorize them right before the exam and as soon as you enter the exam hall, just write it on the front cover of the exam paper. At the end of the exam you can just erase it or cut through. That's what I do to remember formulae when I simply can't memorize them.
I don't know what subjects you are doing, but if its a Science or similar subject, make sure you know WHY things are the way they are. Don't just accept it. For example in Maths, make sure you know WHY the integral of 1/x is ln(x) + c and don't just accept it because the teacher says it.
Make sure you have some images in your mind that tell you how things behave and WHY. A mind map is often very useful. It should be something where you have a
visual image, because people tend to remember that a lot easier than simple text.
I hope it was helpful to you and all others that are interested in finding out how to improve their study skills.
Any questions, don't hesitate to ask
EDIT - btw I just realised how long my answer got. Sorry for the length.