Illustrations, as their name demonstrates, are not the focus of a sermon’s message but serve to help important points of teaching or application be clarified and understood.
They must not distract or confuse, nor should they focus attention on the preacher or alienate hearers from the message.
This article about illustrations by Barry York is a good reminder of some basic principles.
I particularly appreciated this one:
Do not let them be boring or boorish.
If there is one place that should be guaranteed to be more lively and engaging in a sermon, it is when an illustration is given. If a story or anecdote is done poorly and does not hold the interest of the listeners, then things do not bode well for the rest of the sermon. Make them lively both in their content and presentation! Neither should the preacher seek to use the illustration in a condescending way, for instance having as its motivation an attempt to show the superiority of his church over others. Jesus used illustrations to humble pride, not encourage it.
Read the whole article at Gentle Reformation.