Hold a treat in your hand. Use a “high-value” treat, something your dog really loves. Hold it up to one side of your dogs mouth right above the lips. Most dogs will lift their lips on that side in anticipation of the treat. If not, gently brush your hand against his whiskers. This should get him to raise his lip. While doing this, give a happy, upbeat verbal cue…”Smile!” Once he raises his lip, praise him and give him the treat. Once he has accomplished this successfully several times, switch the treat to your other hand and practice this on the other side of his mouth.
Once he is lifting the lip on each side of his mouth consistently, begin holding the treat directly above the center of his snout, over the nose. Give the happy, upbeat verbal cue…”Smile!” As he lifts his lips on both sides of his mouth, praise him and give him the treat.
After he is consistently lifting his lips on both sides, gradually phase out the treats and use only the verbal cue “Smile!”
Remember that every dog is different. Some may show only their bottom teeth while some will show all of their teeth.