There are quite a few threads on this; some people have had success with unseasoned meat tenderizer sprinkled on their food. The ingredient makes their poop unappetizing. There is also something sold at the pet store for this. If you use meat tenderizer make sure it's the unseasoned kind, otherwise they are getting too much salt. For more information you might want to read this:
http://www.squidoo.com/whydogseatpoop They usually outgrow this. Mama did this for them, at least they don't expect you to do it!