My master eye changes from left to right by itself whenever it feels the urge. This can really screw things up when shooting moving targets, so in order to stop the problem I got glasses that are Monovision.
What that means is that in my case, the right lens is set up for distance and the left lens is set up for reading.
So when the left tries to become master eye, it sees a blur and shifts back to my right eye.
Problem solved.