You have good habits and you have bad habits. Brushing your teeth is a good habit. You wake up every morning and you brush your teeth, without having to think of it. You don't stick the tooth brush in your eye, you put it in your mouth and brushing your teeth happens automatically.
Smoking is an example of a bad habit. You weren't born with a cigarette in your mouth, but by practising smoking over and over, you've made it an automatic behaviour. All habits are stored in the unconscious part of your mind; therefore in order to break a habit you have to train your unconscious mind.
Your Unconscious and Conscious Mind
Your unconscious mind is the large, powerful part of your mind that is there to protect you. Your unconscious mind helps you breathe, digest food, and pumps blood to your heart. When a bug flies toward your eye, it's your unconscious mind that tells your eyelid to blink. It does this to protect you. Your conscious mind is the logical reasoning part that thinks things over and talks to people. It can only do one or two things at a time; whereas your unconscious can do many things at any given time.
When you were young you learnt language and now you can speak automatically. You practiced it over and over until it stuck. The same thing is true about any bad habit; you've practised it over and over until it stuck.
Think of your unconscious mind as an elephant and your conscious mind as the rider of that elephant. Once you've trained the elephant to go one way, the rider isn't strong enough to move that elephant based on shear power or strength. The rider is just no match for the elephant's strength! In order to get the elephant to move in a new direction, you need to train that elephant. By training the elephant you're able to break the habit and replace it with a new, better habit.
How to break a habit
In order to break a habit you have to be fed up with it. You have to reach that point where you say; 'enough is enough' and making a change is the only option.
Close your eyes and think of five situations where you've wanted to break this habit. Recall all these situations in your mind as if you were experiencing them one after the other, through your own eyes. Start at the first situation and double the size of the image, turn up the volume and notice those terrible feelings in your body. Move quickly to the next situation and do the same. Go through situations 1-5 rapidly and spin those bad feelings in your body. Keep going from 1-5 as rapidly as you can until you get that feeling of 'never again'. This is usually 'the straw that broke the camel's back' and now you'll feel you can make the change.
Now visualize yourself they way you'd like to see yourself having broken this habit. See the way you look, the smile your face and the way you move when you're free of this bad habit. Make the images bigger, brighter and more colourful as you turn up the volume and notice the good feelings in your body. Notice where the feelings start and where they go and spin them around. Spin them faster and faster until you're drawn to the image of yourself and you desire being this way. Do this for 5-10 minutes until you feel great!
Practise these two visualizing techniques everyday for the next three weeks. When you do this you'll find yourself more addicted to the great, new feelings and images of yourself looking and feeling great. You'll have broken your old habit and given yourself more personal freedom.