Several really good answers. As I wrote about in my book GET REAL, what seems to be effective is to help clients understand that healthy eating habits describe how one wants and plans to eat for rest of their lives, not for just a month or two. It’s why I talk about what I call the 80/20 principle–do what you know you’re supposed to do 80% of the time. 20% of eat however you want to. It’s what you do most of the time that will likely determine one’s nutritional health.
Take care, Daniel