Get The Picture
Once you determine your optimum ball position, the way to preserve it is to take a 'picture' of it.
Set up for a shot, and take careful note of the view you have through your hands to the ball. For me this picture shows the back of my left thumb pad covering the arch of my left foot. When I see the picture, I know I'm in correct alignment and my ball is in its proper position. If I don't see it, I adjust my alignment and/or ball position until I do see that thumb pad eclipsing my instep.
Since I play virtually all my shots from a square stance and with the ball in the same place, this is the picture I always want to see. It stays the same because, as the shaft of the club shortens, my stance narrows slightly, my right foot getting progressively closer to my left as my left foot remains fixed. Try to fix a similar picture in your mind. It's a great way to add consistency and confidence to your pre-shot routine.