I've found that this happens most often in mysql if you delete a lot of rows. So you might want to look at how your system is organized to reduce the deletes or target just those tables that are affected. I probably have 75 tables, but only a couple have regular deletions, so I just optimize those 2 once a week and I'm all set.

__________________
Looking for a unique gift? Send one of our gourmet cookie bouquets today.
Food lovers - visit the Gourmet Gift of the Day Blog for delicious ideas.