It seems to me that if religious or spiritual people are to give up their faith or belief it will not only be hard but leave one with a sense of loss. A good move to decrease these difficulties would be to first move into Agnosticism. How do we make agnosticism more attractive and peaceful?. What slogans justifications and inducements would you use? I think that people get into religion by partly being Hypnotised and definitely by being preached at. We need something more rational and concrete to go the opposite direction.
The question is why do YOU want to have them give up their faith or belief?
For the most part, faith is a cultural component that people are born into. By and large, few people actually choose their belief, and when they do, it is to fill the void of the cultural faith they were raised in. Religion is in essence a form of social bonding, and for the few who were raised without any religious affiliation, their conversion is merely a quest for belonging.
There is obviously an evolutionary component for faith and religion existing in the first place. Agnosticism/atheism is not something to be sold. It is a natural extension of evolution. Our focus should be on protecting that process.
'Agnosticism: because saying "I don't know" is OK.' Haha that was just the first thing that popped in my head. Or maybe it could be something like 'Enjoy life, do what makes you happy, and cherish those around you. This might be your only chance, so make the most of it.'
It depends entirely on how the person became religious, and why the person believes in the first place. I say this because I've run across the whole gamut in my ministry days, and it's mainly for emotional reasons people either converted as adults, or stay in religion because they were born into it. Analyzing which of those emotions are at play is critical to forming a "custom" approach to de-programming. There's no one-size-fits-all answer; it's very different for each person.