I use post-its, and I let some of it stick out of the page in question to serve as a bookmark for important notions. Or if I need more space I write on a separa

I use post-its, and I let some of it stick out of the page in question to serve as a bookmark for important notions. Or if I need more space I write on a separate sheet of paper that I leave in there, at the risk of losing it though.