The most effective form of "external" discipline I ever saw, was by a CSM I had the honour to serve with who, if you were dressed improperly, for example, said "Go and change into civilian clothes. You can't be trusted to wear the uniform properly." On another occasion, when a soldier couldn't keep up on a not very demanding march, he assigned the man two men to help him carry his Bergen. - and they were to do so, whenever a tactical move was in progress. In fact, his whole attitude was to exclude men who could not perform. Shame, not humiliation, was and is the key. Men very much feared being excluded. I used the same basic approach when I was an NCO, and it worked. Boots not clean? Go and get your training shoes.

The most extreme example I ever heard of, was apparently the same CSM, actually sent two men on leave. "Go home. You're useless." I am not saying this is a complete cure, but it worked in a volunteer Army. I only ever saw this approach once, and IMO, it worked.