This is copied from a dog training site and is by Gill White (ttlntl):
Many owners give their dogs treats during the course of the day and when it comes to offering a main meal the dog is very often full and so refuses the food. So, until the problem is rectified, all treats between meals should be stopped. The dog also learns that it worries the owners if it refuses to eat its daily food and gets offered an awful lot of attention for doing so, along with more and more human food in an attempt to entice the dog to eat. This in itself can sustain the dog's behaviour to refuse a lot of the food that is offered. We must also make sure the dog is not getting food from another resource, as this will also lead to it refusing its food when offered.
This always reminds me of the day I was in the middle of a counselling. I came to the question about food and the owner expressed a great deal of concern, saying her dog wouldn't eat anything no matter what they had tried, and had gone without food for over two days recently. I looked at the dog, an English Spaniel, and it looked right back at me and I swear I saw it wink! It was enormous, no way was this dog going without food. I then asked the owners if they had any cats and was told they had six. The story unfolded that the dog was getting the cats food and wouldn't eat its own food when offered. So this is something else we need to watch out for!
Providing the dog looks healthy and normal, then we need to find a well balanced diet, in the form of a complete food, which offers the dog all the daily requirements. You can add some stock or gravy to enhance the flavour if wished, but this must be the only food the dog is offered twice a day. Place the food down for the dog and at the end of 10 minutes if the dog has walked away and left some, pick up the uneaten food and do not offer anything else at all until the next mealtime.