One of my thinking principles is to separate recording of ideas from their evaluation; to separate the process of collecting a lot of candidate ideas from organizing and winnowing them. The former is an expansive process, which includes ideas "popping into my head," and these ideas often occur to me when I don't have the time to immediately pursue them. Yet, I don't want to lose the thought, which is exactly what will happen if I don't do something with it right now. Trying to remember one idea while in the middle of something else is very distracting. So I need idea collection to be optimized for the lowest possible cost of mere capture. The easier it is to get it down, the less likely that I will choose to defer recording it. The system should also allow quick retrieval of recent notes.
For me, the solution I've settled on after years of experimentation is to always carry a small notebook (e.g.Moleskine Volant) that fits in a shirt or coat pocket or a handbag, and write the idea down immediately, without evaluation. Anything and everything goes into this notebook: phone numbers, to-do items, philosophical thoughts, questions to pursue, sketches and shopping lists. Later, usually about once a week, I look over what I've entered, and transcribe the valuable subsets into more durable form. Tasks go into the Outlook task lists and phone numbers into Outlook contacts. Shopping lists and longer essays get retyped into One Note. Now they can be easily searched for. And many ideas, after a week's distance, are not worth pursuing so don't go anywhere. But all get recorded, so none are lost through evaporation.