I tried keeping a notebook for a while (as many of the other replies have said), but I found that it was awkward because either the book would be too big to kee

I tried keeping a notebook for a while (as many of the other replies have said), but I found that it was awkward because either the book would be too big to keep easily in my pocket, or it was so small that writing in it would go through 10s of pages easily, which was then a huge pain to read later. So what I do now is keep 5-10 pieces of normal 8.5" X 11" paper with me, folded into quarters. I put the date and time on the top-right corner of a page when I start writing on it. This makes it easy to figure out the order of pages later, as well as separate out into different projects.