However mistaken a persons conclusions may be, the process which brought them to that conclusion is the key. The important thing is not to focus simply on their erroneous belief, but instead upon what has ultimately brought them to that belief, and then working on getting them to adopt a methodology which relies more upon skepticism, reason, and logic.
This suggests a more modest program than simply trying to convert people: planting a seed of doubt. Rather than attempting to foster a radical change in a person, it would be more realistic to get a person to begin questioning some facet of their religion which they had not seriously questioned before. Most theists whom I encounter are absolutely convinced of their beliefs and take on the attitude that they could not possibly be mistaken and yet still hold on to the idea that they are open minded.
But if you can genuinely open their minds some small amount and get them to reconsider some aspect of their religion, youll be accomplishing quite a bit. Who knows what fruits this questioning might bear later on? One way to approach this is to get people thinking about religious claims in the same way they already know they should approach claims made by used car salesmen, realtors, and politicians. Ideally, it shouldnt matter whether a claim occurs in the arena of religion, politics, consumer products, or anything else we should approach them all in the same fundamentally skeptical, critical manner.
The key once again will not be to simply tear down some religious dogma. Instead, the key is to get a person to think reasonably, rationally, logically, and critically about beliefs more generally. With that, religious dogma is more likely to crumble of its own accord. If a person is thinking skeptically about their beliefs, all you should have to do is point out some key flaws in order to generate a reconsideration, if not a rejection.
If religion really is a crutch, as so many atheists believe, then it is unreasonable to imagine that youll accomplish much by simply kicking that crutch out from under people. A wiser solution is to get people to realize that they dont actually need that crutch after all. Causing them to question religious assumptions is one way, but it is by no means the only way. In the end, theyll never truly be rid of that crutch unless they toss it aside themselves.
Lets face facts: psychologically speaking, people dont like to change or abandon comforting beliefs. They are, however, more likely to do so when they find that it is their own idea to make the change. Real change best comes from within; therefore, your best bet is to first make sure that they have the tools which will help them reconsider their assumptions.