What's the best way to make sure a previous addict doesn't relapse or become addicted to something else?

A big part of helping a person who is an addict is to find out why they are an addict in the first place. Is there something that they might be struggling with that alcohol/drugs help with? Are they feeling lonely? Are they wanting to numb out or escape from everyday life? Often when people think of treatment they only think about what the person is addicted to (like games, alcohol, drugs etc) but they don't look at the reasons why that addiction started in the first place or what fuels it. There's no point in just taking it away because it hasn't solved the problem! We want to help people realise that they don't need to be addicted anymore and start to develop better coping strategies. If we can help them get better coping strategies, then they won't feel the need to get addicted to something else. We can also look at some of the challenges they might face in giving it up (or having less) and conquer them bit by bit - like having friends who don't encourage you to drink for example or realising some of the long-term benefits in quitting.

You unfortunately cannot control an addict or their choices. You can try to help them by giving them healthy coping skills and other outlets for their emotions. In the end however, they must make the healthy choices. You can provide support and encouragement as they seek out the actions that are better for their well being. You can provide them with company and a person to listen when they are struggling with their cravings. You can model healthy relationships and healthy boundaries to give them something to see and practice in their own lives. In the end however, the addict is the one who must take control of their addiction and accept accountability for their choices.