You need to look at what the habit is fulfilling for you. do you smoke to be accepted by your peer group? is it a stress reliever? does it keep you from being bored?
Then you need to look at what you can replace that habit with. Can you exercise for stress? Can you pick up a book to take with you when you're bored?
The last step is to practice your new habit. make sure to get a calendar and make an x on everyday that you complete your new habit. if you can get to a full 30days, you are home free.
I have a friend who used to smoke a pack a day and she successfully quit smoking by going or a jog every time she had a craving. she would let herself have the cigarette after she did her run. she is now an ultra marathon finisher. I think its a great example of what you can do by changing your habits.