I still take lots of paper notes. One thing that had been very useful is to make (short, simple) indices in my paper notes and to digitize just the indices.

It takes almost no time. The result is that when I want to look at old noted I can look up where in my digital indices, and then walk over to a shelf and pick out my notes. I've used this *a lot* and it's surprisingly helpful.

Caveat: I'm a mathematician, so I have a lot of paper notes and half-baked ideas. So my paper notes often look more like a set of laboratory notebooks and not idle learning notebooks. I essentoally never look back on noted I take while learning some new subject.