What is the best way to retain learned materials
This is problematic for students who aspire for higher learning because much of the material or understanding is accumulative.


Actually, that's precisely why it's not problematic.  If you had to memorize random trivia, it would become more and more difficult as you had to remember more unrelated things, but academic studies work in the opposite way.  Because it's accumulative, you are constantly practicing and applying what you learned before, which helps you solidify your understanding.  Furthermore, the more you learn, the more connections you can see, and these relationships help you organize and retain knowledge.  Of course you'll still forget some details, but you'll forget less than you expect and recover it more easily.

My impression is that severe forgetfulness in students typically occurs when they are studying ideas in isolation and setting them aside as soon as the class is over.  Instead, it's important to play with ideas constantly.  How are they related to your past studies?  To other interests of yours?  Can you think of further applications or connections?  This can help bridge the gaps between how you learned these ideas.  This sort of exploration is almost essential if you want to do research, and it's a useful study technique in any case.