I read an article from a family psychologist who writes a parenting column who said to tell the child that the DR. said he has to stay in the bathroom untill he poops, then leave him in the bathroom, gate him if you have to, but tell him that the DOCTOR is running the show and he can't come out untill he poops. I know it sounds kind of cruel but it's supposed to have very high sucess rate, with most kids completely toilet traned in 1 week. This is only for kids who "poop hold" and you should see the peditrician 1st to make sure there is no medical reason. John Rosemond is the writer, his email is www.rosemond.com/parenting, I'm sure if you write you could get the whole story, he just ran that article about a week ago, I read him all the time because his advice is usually right on, no frills, get the job done!.........Good luck....C., mom of 3.
I just read some of the other responses which are mostly positive rewards, which can work, but haven't as you mention. I guess with the approach I wrote, it takes the "bad guy" away from you (the parents) and puts it on the DOCTOR, so the power struggle is not supposed to be a problem, think back when you were a kid, if the Dr. told you to do something, you did it. Come to think of it, if you're PARENTS told you to do something, you did it!!