Understand the nature of addiction that the person you are trying to help is recovering from. Having a general idea about the nature of their addiction will allow you to prepare yourself better to help them. Do not feel sorry for them or get mad at them for developing an addiction in the first place.
Help the addict by refusing to help them. Do not take this wrongly. The person you are trying to help recover from his addiction is bound to beg you to return to the thing they are addicted to just for a little while. You may see them suffering, or come to you to ask for money to buy food, clothes or simply necessities for their household. Do not let your emotions cloud your mind as the money you give them will most likely be used by them to buy drugs, alcohol or whatever it is that they are addicted to. Instead of helping them, you will be pushing them back into the quicksand that they are trying to get out from.
Never believe a word that an addict says, especially when it has a hint of self-pity in it. They may very well be trying to deceive you or making you weak by tapping into your emotions. Addicts are really good at deception and manipulation, having perfected it during the days when they had completely given in to their addiction. If the person promises to quit after just one last drink or shot of drug, do not be fooled. You should only believe what you see with your own eyes, i.e. their actions instead of what they simply say. Deception and manipulation are a strong and effective tool, which will require you to stay on your toes around the clock.
Be firm and set solid boundaries. While recovering from an addiction, the person is bound to throw a lot of tantrums when they have trouble accessing what they are craving for. They may become rude to you and may even physically assault you. At other times, they may simply refuse to eat or drink anything, stubbornly demanding for a break from the recovery process to feel better. However, do not give in to the pressure and continue to make efforts to keep them away from their addiction. This will show them that you really care about them and want to see them return to an addiction-free life.