How do you reach out those Mosaics (Gen-Y)? Thankfully, a great ministry in post-christian London has been doing that for a while. It's called the Damaris Project (Link above). I think of it as L'abri 2.0. I don't say that lightly. The director is an evangelist named Nick Pollard. The site has different articles dealing with ways to use the current culture to reach people and some great examples of how to do that. Furthermore, they strive to be biblically faithful and this is material you can use with middlers, high schoolers and older college students. Nick has beed doing student ministry for years in London colleges and definitely knows what he's doing.
His book "Evangelism made slightly less difficult" is a great read. He gives you a tool he calls positive deconstruction. You talk with the person you're witnessing and slowly help them unpack their world view and you begin to deconstruct it to show them their need for Christ. I hope to share more on this. It's all very schaefferian. Visit the site and book mark it. You'll be going back again and again.