Addiction is a mental condition of a living being in which the body of that living being craves to do some act or to have some thing. When you cannot stop yourself from doing any act in particular then it means you are addictive to that act. There are many kinds of addictions. Most common of them are drug addiction, smoking addiction and sexual addictions. Almost all kind of addictions have a cycle like scenario before you act them out.
First your start fantasizing about doing the act you are addictive to, then in second step, you become excited about it, in third step, you become some much excited that you ignore any thoughts being generated by your mind against that act and start to do that act, in the last step which is actually after doing that act, you become guilty on your act and promise yourself that you wont do it again. But once the cycle is started again, you are again pulled to the last step.
Here is the addiction cycle diagram which visually describes all the steps.

Addiction Cycle Diagram
Alright, now when you have seen the addiction cycle diagram and have realized what the problem indeed is, It is much easier to fight with your addiction. Find the weakest part of this addiction cycle and try to break this cycle at that point. Under my experience, the weakest part of addiction cycle is the one between fantasizing and excitement. You might argue that the weakest part is between guilty and fantasizing but the answer is, you cannot stop the first thought but you can control the further thoughts. For example, if you are smoking addictive, whenever you will see some one smocking a picture of smoking will be drawn by your mind, you cannot stop it. However, you can try to stop further thinking about smoking and thus can stop the excitement building process. This can be achieved by engaging yourself in some other activity, like going out for a ride in an area where no cigarette shop is nearby. You can visit those friends who don’t smoke and can spend some time with them.
Similarly, if you are addictive to alcohol, try to read medical facts about alcohol. You will find out that alcohol is very dangerous for your liver and can cause an early death. Alcohol can harm you medically as well as socially. When you will read about the side effects of alcohol, your mind will slowly develop a barrier against the excitement for alcohol consumption. Remember, what you see is what you believe. Whatever you read or see is seamlessly stored in your mind and affects your daily life. Try to gather negative facts about your addiction and read them on daily basis. This will help you in breaking your addiction cycle.