Having spent a lot of time in doctors' clinics and hospitals, I was quite surprised the first time I saw doctors as people - drinking, smoking, enjoying life as much as any normal person would. It was during medical school when I realized that doctors are people too.
I remember getting a glimpse of our family doctor on my way home from elementary school. He was playing mahjong. Needless to say, he was smoking and he sounded far from the mild-mannered doctor that I knew him to be.
In medical school I saw that doctors are not any different from the ordinary person. Having an M.D. does not place you on a moral high ground, nor does it enable you to preach.
As a patient, I tried to rationalize if the doctor's advice is doable and practical. For the most part, it is. I brought this line of thinking when I had the credentials to sit on the opposite side of the table - will I be able to follow my own advice?
If I'm not able to follow my own advice, I don't use the authoritative tone that we are expected to utilize in "educating" (or, more appropriately, scaring) our patients. I change into citation mode.
Citation mode is a trick I learned from one of my senior residents when I was still a clinical clerk. You should at least try to vaguely cite a valid scientific reason for the drastic lifestyle change you are proposing to your patient.
For instance, I am no stranger to alcohol (but I do drink only on occasion). Normally, it would be hard for me to chide a patient for having a habitual drinking problem. Doubly hard if the patient knows me personally and have seen me drink at one such occasion. Citation mode would require me to pull up some facts about the chronic effects of alcohol, translate them into layman's terms, and tell the patient those converted facts. It works for me, educates your patient instead of scaring them, and places your treatment relationship into a realm of increased trust and cooperation.
The citation mode would work for you too. I still practice it, even if I am limited to the usual complaints of upper respiratory tract infections and sleep disorders in my new workplace.
I think that this approach of slightly depersonalizing your medical advice by supporting it with fact or current scientific information is the solution to the dilemma of doctors not being able to "practice what they preach". Educating and empowering your patient is the new paradigm for medical care in this era of multiple information sources. No longer is the doctor the "master of the ship" - doctor and patient should look at their relationship as a partnership with shared trust and responsibility.