Sermon illustrations are servants, not masters. They should not rule the sermon, but should serve it. Commenting on the value of myth, C. S. Lewis said
"it takes all the things we know and restores to them the rich significance which has been hidden by ‘the veil of familiarity.' The child enjoys his cold meat (otherwise dull to him) by pretending it is buffalo, just killed with his own bow and arrow. And the child is wise. The real meat comes back to him more savoury for having been dipped in a story; you might say that only then is it the real meat . . . By dipping them [realities] in myth we see them more clearly"
Quoted in Jerram Barrs, Echoes of Eden, 135
This is the purpose of sermon illustrations: to dip the realities and glories of the Bible into a story, a word picture, a quote, an analogy, or a personal example. But we should never lean too heavily on them or allow them to take center stage in our sermons.
How do we make illustrations serve, but not master, our sermons?
Let them come naturally
When someone asks me how long I spend preparing sermons each week I am tempted to reply, "Remind me how many hours there are in a week?" The fact is, I'm always preparing. I constantly read, listen, watch, and think with an eye toward the sermon. This way, illustrations remain within my grasp.
If you burn up time each week looking for illustrations, you will inevitably place distance between them and yourself. They won't feel natural, but will seem canned or forced. Illustrations should flow from your reading, listening, and thinking. For them to impact your hearers, they must first impact you.
Use them sparingly
Many preachers feel pressured to include multiple illustrations for each main point. But too many illustrations can create unnecessary separation from the text. It's like trying to eat ice cream that's still wrapped in cellophane. Simply make your illustrations serve the main idea of the sermon. If an illustration stands the risk of adding complexity to the sermon, toss it!
This goes especially for visual aids. Video clips, pictures, slideshows, or tangible objects should, in my opinion, be minimized in sermons. While these things often appeal to a room full of people, they may in the end hinder the clear exposition of the Word of God. His Word, not illustrations, nourishes our souls.
Use stories carefully
Stories are thieves. They tend to steal the sermon spotlight, so handle them with care. Don't use too many in one sermon. If you conclude with a story, make sure it doesn't rob the glory of the text. As for personal stories, proceed with caution. Your people need to see your life, that your marriage, devotional life, parenting, and inner struggles are as real - and as messy - as theirs. But the dangers here are self-glorification on the one hand, and distracting your people from the text on the other. Work hard to keep these in check.
Use them accurately
If you tell a story about yourself, be truthful. If you quote an article, be sure you've read it. If you defend the claims of Christianity and refer to a different religion or system, present that case fairly and consistently. Don't simply knock down straw men you've set up. Be accurate, balanced, and clear.
Use them humbly
We preachers are tempted to use anything we think will get a rise out of our listeners. Many of us have crossed the line of propriety just to get a laugh, shock our listeners, or receive an emotional response. The Gospel is offensive enough. Your illustration should not add to it. If an illustration will inappropriately offend someone, don't use it. If it will make much of you at the expense of someone else, don't use it. If it will belittle the opposite sex (especially your wife or daughter), don't use it. If it will cause undue controversy in the church, or for you, don't use it. If you are uncertain whether it crosses the line, don't use it.
Every preacher is different, and every sermon requires wisdom to know which, and how many, illustrations should be used. Discerning whether or not you are leaning too heavily on them is difficult. One question to ask: "What excites me the most about this sermon?" If it's the illustrations you've crafted, you are likely leaning too heavily on them.
Preach Christ. Preach the Bible. Preach the gospel. Make your illustrations serve the truth and glories of the text. Illustrations are servants, not masters.