My family had a drug addiction intervention for my cousin. It didn't go too well. He denied that he was addicted and blamed his parents. They didn't push any further when he reacted this way. After about five months, my cousin approached his parents on his own and said he wants to be treated.
From my cousin's experience, I think in order for the intervention to work, it's important to get professional help. We become emotional when the addict is a family member and we really don't know enough to plan out what we are going to do if the intervention works or doesn't work. So I think that getting a professional's help was the first thing that my family did right.
The other aspect is that we did our best to show and tell my cousin that we love him and want to see him happy and healthy. If an addict believes this, I think it helps them come to terms with their condition more easily.
The other thing, which is not in anyone's control, is for the addict to be ready for treatment. At the end of the day, if they are not ready for it, it won't work. We could have pushed my cousin to be treated sooner, but if he wasn't ready, he could have been treated and still continue his addition after he came out. I wish we could do more to help them be ready for this change, but it's something they have to do.