Besides everything else mentioned here, try adding unsweetened pineapple juice to their meal. Pour a bit over their meal. Dogs usually love the taste of this juice on their meal BUT when it passes through their digestive system, the resultant stools are very, very unattractive. Once bitten, twice shy from then on.
This tip is a favourite on the Pet Vet Radio Show with Vet Rob Zammit in Australia. (2GB Sydney on a thursday night about 8PM).