As long as the needs of the client are met, posthypnotic suggestions will stick and eventually become permanent.
A smoker who smokes when feeling stressed is given a posthypnotic suggestion to find a more pleasing behavior every time stress occurs, so instead of wanting a cigarette you'll just have the desire to get up and stretch, take a short walk or go have a glass of water or whatever other substitute behavior was suggested that suits you best. The mind will not accept suggestions that don't fill your personal needs.
You cannot be hypnotized against your better judgment so you can't be hypnotized into doing things that aren't good for you.
