I use mnemonics like memory pegs when I need reminders. I like to be freeform so I also have several small paper notebooks going at once. Daily journal entries and longer stuff goes in markdown files in folders on Dropbox. I like Regexxer or Grep or ag for searching those. Google Keep for longer notes if on my phone. Hi-Q MP3 Recorder for Android if I need to do voice recordings. I don't like transcribing those voice notes later but the recorder software is really nice.