What is the best way to retain previous learnt materials

No matter who you talk to, students or professors, they all say that the material they once learnt can be quickly forgotten without further experiences that utilizes or enhances that experience.

I fondly remember a professor once telling me that him final year in undergrad was the happiest year in his life, when he felt he could do anything, from nuclear physics to parallel programming to electromicroscopy to constructing an audio amplifier...then one year later he has forgotten almost everything.

This is more noticeable in students, where it is often exaggerated to the cliched phrase "you never actually use anything you learn in school".

This is problematic for students who aspire for higher learning because much of the material or understanding is accumulative. It amazes me how people manage to get all the way to the top of the academic ladder (PhD, Post-Doc) without losing previously gained knowledge along the way. I'm sure there are important theorems, relations or techniques that I once learnt, maybe was even an expert in, the question is how do I unlock these memories so I can be more effective in tackling the problems I have today?

How do people deal with the invetiable loss of knowledge from years of disuse? Do you start from scratch? Can someone offer good ways for retention of class room materials?