I now think the key with www.introducinggod.org (and other courses) and why we (at Village Church) have seen and do see a larger percentage of new Christians is that we just keep running small courses, and then an occasional larger course (that gets the whole church focused again on reaching their friends).
We are starting a new Introducing God course roughly each term (with each course evolving into a more conventional Bible Study group). What we find is that roughly half the guests who complete the course have trusted in Christ by the end of the course, with the other half trickling into the kingdom over the next six months or so.
So if there is a key or (I don't like to say 'silver bullet') it is to 'just keep running courses.' Sometimes they will start with ten guests, some times they will start with three guests. But just keep starting courses.
It forces me and the others in our leadership team to be looking around at least once a quarter to be thinking who is on the fringe of our relationships that we could invite along.
[How do you find guests] Well, in first term - it's predominantly following up the relationships of the dust stirring from our various Christmas events, in second term it's people we have come across through the dust stirring of Easter. Then it's the general fringe of church - most of the invites have been friends of evangelistically entrepreneurial members and staff."