good question...i am also interested in knowing the answer for this.
Write things down. Break it into smaller parts and then learn. You can also make a learning board, so that you can look at it regularly and remember things.
Use mnemonic devices. And mediate for increasing your concentration and retention in the long term.
It might help to try and figure out what kind of learner you are. I, for one, learn better when I hear things: so I read aloud what I want to memorize. Some people learn better by writing out what they want to remember; others rely on images so they convert the material into figures, diagrams, flow charts, etc. See what works for you!
In addition to the above suggestions Memory consolidation happen best in sleep, so something you find difficult to retain or to memorise, try and read them right before sleeping and the first thing you do is read the content again once you are up, try it for a few days, it shall to wonders to your memory retention.
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If it is the formulas you are asking about,the best way is to solve questions related to that formula. Solve 3-4 questions on the same formula,if there aren't many questions on that then repeat the old questions after few days.