For Mathematics, the key is to practice. One should practice at least 15-20 problems on each sub topic of a chapter and then previous years’ questions. If you’ve practiced enough number of questions you’ll remember the formulas and the concepts. There is no point in sitting down and try to by-heart the formulas as exam problems will not ask you to write down the formulas but how to apply it. Hence the key for Math is solving a lot of problems.
For Physics, remembering the formulae or a particular concept does not guarantee that you can solve the problem. There are thousands of concepts and very few formulae. Every new question that you come across seems completely different. Hence for Physics one should concentrate on the concepts more than the formulae. You cannot solve all types of problems of a chapter in Physics because there are too many different types of problems, so concentrate on how the last problem was solved, remember the concept and go through each step very carefully and try to understand the approach for the step instead of the calculation.
For Chemistry, major problem one faces during an exam is trying to remember Chemistry topics, especially Organic Reactions and Inorganic. Chemistry concepts are easy to understand but incredibly tough to remember because of the sheer quantity. A common mistake that most students do here is their approach to revising a concept. What students do is that they keep their notebooks open, read something (like a reaction), and say it in their heads and keep re-reading it multiple times. This approach is extremely time-consuming. The best way to learn reactions is this – Read a few reactions (say 5-6 Carbonyl Reactions), try to remember them and then keep the notebook away. Now whenever you have 5-6 minutes free during the day (like when you are travelling or sitting and watching TV), take a piece of paper and try to write the reactions (with the steps) as much as you can. You obviously won’t get everything correct. Even if you don’t have pen and paper just try to remember it in your head. The key is to not use the notebook at all. When you are done, get the notebook and see where you went wrong. If you do this 3-4 times, the next time you’ll be able to write the reactions (with their names) easily. This way you will not waste hours and hours of precious time in trying to remember and your brain will have enough time to freshen up, as anyone can tell you that you can’t learn 100 reactions in 2 days.