I have been taking (plain text) notes for many years. My approach is to write down immediately everything I figure out. The biggest issue I have is to retrieve notes effectively. And to be faster creating them. I started experimenting with how to make the whole process better and have built http://codingbrain.com. It's basically a better search, copy-n-paste, history tool that works on local markdown files.

My other idea I'm currently working on is to provide a different kind of bookmarking service that gives you an option to write down what you have learned from that bookmark. I think one of the best way to get better is to always try to repeat from memory what I have learned. The service is supposed to compare my summary with the original content, maybe offer some Anki style repetition training etc