Colin Adams on his Unashamed Workman blog has a helpful post on “The Seven Commandments of Illustrations.” Make sure that you read his entire post, but his seven commandments are as follows.
1. Thou shalt not overuse them.
2. Thou shalt not underuse them.
3. Thou shalt not fail to illustrate the point.
4. Thou shalt not make them overly long.
5. Thou shalt not misuse humour.
6. Thou shalt not be pastorally imprudent.
7. Thou shalt not use overly powerful illustrations.
Also on illustrations, Peter Mead at his Biblical Preaching blog notes that illustrations should serve the sermon not vice versa. He writes:
“Any trained public speaker can select a theme and gather a bundle of stories that will touch an audience emotionally, but this is not preaching.” (Chappell, 200.) We need to remember always that an illustration is there to serve the sermon, to aid in clarifying explanation, support, or application, but not to substitute for sound explanation.
If you suspect that a message might be too illustration-heavy (a rare problem for some preachers), then it is worth going through the message and questioning the purpose of each one. Is it there to clarify explanation, to support a point, or to apply the teaching in real life imagery? Or is it there because you really want to tell it, or because you know they’ll enjoy that one? Be ruthless in filtering illustrations so they are genuinely serving the sermon.
If people perceive you to be a preacher who just tells stories, then your credibility will be damaged. Be sure the illustrations are the servants, not the focal point of your preaching.