Though this concept may take years to fully internalize, we can start trying today! The first step is to put down the notation! Notation is a great way to learn and remember new songs, but it’s easy to become dependent on it. Something people will use even if their hands have already formed muscle memory.
Again, notation has advantages, but once you’ve memorized the song, it’s beneficial to stop using it and focus on the essence of music, feeling and flavors!
So the first step, choose a song you can play comfortably. For example, Sakura, Kuroishi Yosare Bushi (from the Crash Course ), Happy Birthday, or anything you can play comfortably without notation.
Next, play your memorized song a few times, listening to the tone of your strings. As you play, move the bachi closer/farther from the bridge and notice how the tone changes. If you’re experienced, use techniques like sukui with varying degrees of force in the pullback and notice how that affects the tone of your shamisen and the feeling of the song in general. Most importantly, don’t worry about playing anything “wrong”. Just observe what you play and the reaction you feel afterwards. (Just like when I tell dirty jokes at the formal ball!)