Sometimes the key to stopping a particular activity is replacing it with something new. If you are accustomed to smoking at a certain time everyday, maybe during your coffee or lunch break at work, talk a walk outside instead. Not only will this help with breaking bad habits, but it may result in the extra benefit of you getting in better shape. If you arent feeling energetic you can always use the time to read a book, or make a list of things you need to do. Anything that can take your mind away from the habit you are hoping to break is a step in the right direction.
Another factor is will-power misuse. Will-power should be channeled to the correct destination instead of being wasted somewhere else. Will-power misuse is like trying to break through a wall instead of opening the door using the knob. An example of will power misuse is when you suddenly become very enthusiastic then run to do a certain task until you get bored. To channel your will-power correctly, write down a plan when you are enthusiastic instead of quitting a habit for three days then returning back from where you started.