I have been guilty of a few bad habits in my lifetime, as most of us have been. The two most distinct and long running habits that I had were smoking and nail biting. Both of these habits were rooted in stress, which is one of the two main reasons people start a bad habit. The other main reason is boredom, which may have played a role in embedding smoking and nail biting into my routine as well.
All too often people try to quit bad habits like these cold turkey. This is not usually the best method, as there is nothing to fill the void of this constant repetition in your daily cycle. Whether you habit is a crutch, or whether it is just a routine you have grown used to, your mind has a place for it and just ripping it away can cause a sort of “withdrawal”. This can be especially true in things that have addictive ingredients in their hold on your habit, such as nicotine in cigarettes.
Most people, myself included, are usually more successful in their crusade against bad habits if the replace them with a good habit. Studies have proven that it takes longer to break an existing habit than it does to ingrain a new one into your routine. So if you think about it in this way, you can replace your bad habit with a new “good” one easier than you can try and undo what you have already taught yourself. Also, a lot of bad habits have certain triggers. These triggers can be as simple as smoking when you drink or even just lighting up a smoke after every meal. If there is a trigger area, this is something you will want to avoid, or even place your new substitute habit wherever this trigger may be. For example, you may have a cup of tea after every meal instead, where you may have otherwise gone out for a cigarette or cigar.
Seeing the effects that smoking had on other members of my family was enough motivation for me early on to kick the teenage habit of smoking that I had acquired through peer stress and boredom. I was able to replace it easily with other activities that did not have the negative health effects that a continued bad habit of smoking would have had if I had continued. Nail biting for me was the worst 
habit, as I started in my childhood and continued for about six years. It was to the point I would bite them until they would be bleed and became sore. I was able to do the same and replace this habit with other things until I no longer felt the urge to continue that disgusting habit. I am a firm believer that the human mind can conquer anything if approached from the right angle and plan of attack. Be sure to surround yourself with positive, like-minded people that will help you along your journey to developing healthy new habits to replace your bad ones.