This is the trick that people ask about the most so I thought I would explain it first! Ever since Cooper was a young puppy he has had his tongue sticking out some of the time. When he was 3 months old I decided to turn it into a trick. I thought it would be fast and easy since he did it naturally and I could "capture" it using a clicker, but I was very wrong! His tongue was hanging out subconsciously so he wasn't aware of when it was out or in.
In order to get him to recognize when his tongue was out, I had to do a LOT of repetition. I sat with him for hours and hours and clicked and treated him every time his tongue poked out. I clicked him hundreds of times over a span of 2-3 days. On these days I couldn't give him regular meals because he was getting so much kibble during training! His training kibble had to be his meal.
At first he kept running through his repertoire of tricks trying to figure out what he was doing to get the treat, but eventually, on the second or third day, his tail started wagging when I asked him to stick out his tongue, he stopped running through all of his tricks, and he started consciously sticking his tongue out on command. I knew he was doing it intentionally because when I asked him again he would poke it out farther and farther. He also did it immediately after I asked, and on previous days his tongue just came out randomly. When he was a few months older I also taught him to stick his tongue back in on command. I used the same method for that - clicking and treating him when his tongue went back in.
I don't think this was a difficult trick to teach. It just required a lot of repetition and a lot of patience - from both me and Cooper. If your dog doesn't naturally stick his tongue out sometimes like Cooper does, I would recommend using peanut butter or holding a tasty treat in front of him to make him lick his lips.
Supplies needed: patience. Supplies recommended: clicker, treats/kibble.