My introductions are often preparatory comments, apologetic reflections, related topics that-we-don't-have-time-to-explore-today, broader systematic categories within which the exposition sit and so on. I trust that I can do this in a signficantly interesting way that you'll want to hear the rest of my sermon.
My friend Des Smith tends to give big illustrations in his introductions. But they are not illustrations of the 'big idea' of the sermon, but rather the big idea that was going through Des' mind as he was writing the sermon.
Mark Driscoll tends not to have introductions. He just starts talking in that husky phone voice of his and we're all ears.
Phillip Jensen says that it's good to find something intriguing in the text of Scripture itself. That way, from the very beginning you are grabbing people's interest with the Bible.