All devotees of the Hare Krishna movement aim to strictly follow four rules
In Krishna consciousness, you're trying to purify your mind--to make it clear and steady for spiritual realization. But gambling makes the mind cloudy. It agitates the mind. You start thinking: What are my chances? How much could I walk off with? What could I do with my winnings? Even if you just play for the sake of playing, gambling pulls the mind away from thinking about the real purpose of life.
No drugs, no liquor, not even coffee, tea, or cigarettes. The world is already high enough--high on illusion, high on greed, high on false power and false ego. The devotee of Krishna wants to be thoughtful, sober.
Material substances like drugs are useless for spiritual realization. They can bend the mind, but they can't free it. They may take you up, but sooner or later they let you down.
The self within is by nature joyful. So we don't need any material stuff to *make* us happy. We just need to purify ourselves and uncover the happiness that's already there.
No Meat Eating
No meat, no fish, no eggs. Devotees of Krishna avoid needless violence. They see other creatures as spiritual living beings. Though these creatures may be less intelligent than we, they are also God's children, our brothers and sisters. So why should we kill them? If a child were to kill a less intelligent brother, do you think the father would be pleased?
By the subtle laws of karma, the suffering we cause others will come back looking for us.
By nature's arrangement we have plenty of fruits, vegetables, grains, milk products, and other innocent food. We should content ourselves with this and be happy.
No Illicit Sex
Everything has its place, and that includes sex. When sex takes place within marriage, and for the sake of having children, it's fine. You're accepting the pleasure, and you're accepting the responsibility.
Otherwise, sex brings about entanglement, exploitation, disappointment, and illusion. For spiritual advancement, you want to simplify your life and bring your mind and senses under your control. Being temperate about sex helps you.
(Extract from Hare Krishna Movement Blog)