I recognize that there are multiple legitimate ways of “illustrating” a sermon, although I suspect the helpful options are sometimes more limited than we might imagine. I sometimes prefer to think in terms of explanations, proofs or applications rather than the more generic term “illustrations” (which can and does slide into time-fillers, interest-adders or expected-anecdotes . . . all of which I would resist). To simplify things, I think there are essentially two sources of helpful “illustration” that we should always look at.
1. The Contemporary Life Example. How did Jesus illustrate? Generally not with other biblical passages/stories (and this to very Bible aware Jews, totally unlike the increasingly biblical illiterate listeners of today). Nor with historical examples (and this to a very historically oriented people). But with everyday examples that listeners could easily relate to. Good illustrative material comes from the everyday experiences we can describe and use to help people to understand biblical truth, or visualise themselves applying the message.
2. The Inherent Textual Imagery. Generally speaking, Jesus was teaching new and direct truth, we are teaching Bible passages. So the other main category of “illustration” material is the imagery right in the text itself. Help people to see what the passage is saying (whenever possible use the imagery implied by the passage itself rather than rushing to another passage, or rushing to some “interesting” extra-biblical material).