1. Talk to your dog in a kind and loving manner
It takes a happy dog to be able to smile, the same as people. Therefore, it would help, while during training that you are smiling too, although avoid baring your teeth when smiling, as dog's can see this as anger, aggression. Which could lead your dog to believe although you are petting it, it wouldn't want to step out of line and smile.
2. Show them how to smile by using your thumb and index finger
Gently push each side of the dog's mouth upwards, so that the teeth are showing. While you are doing this, say to the dog "oh, look you're smiling." Repeat this step, several times, a day, for about a week or so, depending on how fast they learn. Award the smile with treats and lots of affection.
3. Get your dog to smile by asking your dog "Can you smile for me?"
You can even use the word mom or dad in place of the me word. Since, many of us relate to our pets as being their parents.
http://www.wikihow.com/Teach-a-Dog-to-Smile
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