I know little music theory, but I've been playing instruments "by ear" for a long time and it made many things easier to me when I learned that if you're using

I know little music theory, but I've been playing instruments "by ear" for a long time and it made many things easier to me when I learned that if you're using diatonic scale, it's very unlikely that you'll use notes outside of the scale; e.g. if you're playing a song in C Major, most of the time you'll play C and D, not C# or Db, so if you need that note, it's easier to switch than having an almost unused line in the sheet.