One of the principles of using an illustration is to serve the text being preached. As Bryan Chapell says, "The proper focus of illustrations lies in presenting biblical truth in such a manner that it can be understood deeply and applied readily, rather than in providing popular enjoyment or pastoral acclaim". Chapell also warns that, "the preacher who constructs sermons to serve illustration rather than solid biblical exposition inevitably drifts from pulpit to stage, from pastor to showman".
I know that I have overused an illustration if somebody remembers the "parakeet story"* without being able to match it to a biblical text or principle. I can remember some of Aesop's Fables--but I also remember the moral of the story. Do your hearers remember the story or do they remember the principle the story tells?
This may be an example of over doing an illustration: Pastor Injured in Dirt Bike Accident During Church Service.
*There is no parakeet story--I just use that as an example.