Well, here’s how my teacher would have forced me, if necessary: not letting me play a single note with a different finger, stopping me, holding my hand if necessary so it comes out right. That is how I learned good position and technique, and it was the only thing that worked for me. Holding the student’s hand is essential, basic, primal, and most effective. I have tried using a conductor’s baton, which is good for reaching between the strings, but it was a little too pointy, perhaps.