You stand in front of your horse, point to his nose, tell him smile, and he will lift his lip in the flehman response as a smile.
Cautions: You might want to wear gloves while teaching this trick. Use larger treats rather than smaller ones.
Tickling nose method: You can start this trick standing next to your horse. Hold the treat just out of reach of his upper lip. Your goal is to tease him with the treat to get him to reach for it with his upper lip. If he does this, even a little, reward. Ask him again to reach for it with his upper lip, but don’t reward unless it is as good or a little better than before. Repeat this, gradually increasing the difficulty, as many times as is needed. As he begins to understand, start adding a verbal cue, such as “Smile,” to let him know what you want.
Once your horse is starting to really understand what you are asking him to do, go stand in front of him. Point to his nose and say “smile.” He probably won’t do it on command yet, but if he does: reward! If not, use your finger and tickle his nose like you did when you had a treat. If he lifts his lip a little: reward. If he doesn’t, go back to using a treat. It will take a little time, but you can begin weaning him off the food as his cue. With practice, all you will have to do is stand in front, give him the cue, and he will smile. Then work on getting him to hold the smile longer and longer. To do this, simply reward only the times when he holds it a little longer.
The important thing is to keep things fun. If your horse seems to stop trying, perhaps you should call it a day or work on something else. You can teach this trick with a clicker or without.
Trouble shooting: Your horse won’t reach for the treat with his lip: Make sure you are using food that he normally loves to eat. If he is just turning his head away all the time, get a halter and help him keep his head there while you play with his nose.