By no means can any advice on addiction be a cure-all, mostly because the subject is not at all so cut and dry. Many people seem to suffer from addiction at some point in their lives, wheather it be alcohol, harder drugs, cigarettes, or just plain old caffiene. There are also a surprising variety of addictions related to subjects other than just substance abuse.
To those who struggle with addiction, big or small, or care about someone who does, knowledge can be a comfort. In this instructable, I give you one person's knowledge on the subject, and attempt to break it down; cut and dry as possible.

Step 1: A Need Unmet

Simply put, our basic needs as people can be summed up into four categories.
Physical
Mental
Emotional
Social
When a person is unable to fill a basic need in their lives, it is a natural instinct to focus on it and try to fill it, at any means. In today's life, and with the cultural independance of individuals in the UK/US, many people can end up feeling lonly and isolated. With the average modern diet, many people can end up feeling physically unhealthy. Boredom, lack of mental stimulation, sense of accomplishment, social interaction, and/or physically healthy habits, can potentially leave practically all of a person's needs with gaping holes. Trauma is another emotional category all in its own that can leave a person with thoughts and ideas that discomfort them immensly.
These gaping holes in a person's needs scream to be met.
When a person can not meet them by filling them in a healthy way, they may seek alternative solutions.

Step 2: At First

Whatever the addiction may be, the reason for it's appeal is that it seemingly fills a void. For a short period of time, it can mask the unmet needs that a person feels, and give them a sense of fulfilment.
A common cigarette holds the same principle as cocaine by these means.

Fulfillment, or accomplishment, is one of the most potent drives of our human brains. When you are hungry, and then eat something, your brain releases 'pleasure' chemicals. When you finish a workout, have a good conversation with someone new, or complete a school paper, your brain sends out these same 'pleasure' chemicals. In fact, a lower percentage of people were depressed durring the time period of the great depression than today. This is said to be because people worked harder and had a greater sense of accomplishment from daily tasks. Bread on the table, earning pay, getting through the day. Today, it's point and click, and if we don't get what we want immediatly, things are bad. There is no work involved with many things that there used to be.

It is a very primal and often irresistable instinct to do what gives you pleasure again. Makes sense, right?
Now, wheather the drug itself is designed to release pleasure chemicals (like most hard drugs,) or it is just the sole sense of accomlishment of attaining and preforming something associated with pleasure, this is the beginning of addiction. The brain begins to associate the substance with pleasure, and a primitive being takes over. One becomes tunnel visioned.

Step 3: Stage Two

After using this substance to a point, it becomes not a mask anymore, but another need to be met.
For some ammount of time, this can seem helpful if the other needs in this person's life are still not being met. But at this point, even if things have changed for the better, the addiction doesn't just disappear.
Well, why not?
After preforming an action however many times, a person's physical makeup of their brain begins to change. For the same reason that practice makes perfect, addiction is a hard thing to forget. New, physical, pathways of thought form, and the only way to get rid of them is to be actively involved in telling them 'no.' Associating the thoughts with shame, guilt, and other negative things that stop this train of thought in it's tracks make it less of a 'pleasure.' These memory cells, after being unused for a long period of time, eventually weaken and die. It is a gradual process.
For a more intricate explanation of how the brain creates these 'pathways' of thought, there are multiple studies on the subject. Scientific american has a few good ones.
http://www.scientificamerican.com/article.cfm?id=cocaine-addiction-stems-from-desire
Once again, this explanation is the same for caffiene, tobacco, and illegal substances alike. The difference is in the severity.

Step 4: Conclusion

Now that you know the problems, a solution can be deduced.
Firstly, the unmet needs that begin the intial desire for something else need to be met; sometimes 'and some.' A reason why some recovered addicts choose to become religiously involved is because, like substance, a higher power creates a '5th' need, but this one does you no harm. In fact, it can help quite a lot. This can create a mental cusion for when things get tough. Instead of substance filling the voids, religion then can.
Addicts need to be actively involved and motivated in order to rewire their brain. Not to say that others can't play a large role in pushing them in the right direction, but others can, but no means, do it for them.

In the end, nothing is a real substitute for the basic needs of a human being. Anything from a bothersome habbit, to so much suffering can be brought on by such a simple seeming thing like a substance. Take comfort in your knowledge, and seek/be of a healthy support to your loved ones. Strength is a virtue, and a better day can come.