NOTE THIS TAKES WILLPOWER ON THE PERSON DOING THEIR PART
I did smile a little in the past, but I worked on myself enough that I was able to make a noticeable difference that a cousin of mine commented on my improvement.
I was going through some hard times. I wanted to be in a different enviorment, one where I would have more of an oppurtunity to learn about my Jewish heritage (because I realized Hebrew school taught me nothing)). But then I realized why I was feeling so bad. I was focusing on the negatives in my lifes and not on the positives. I worked on being more grateful and started a gratitude journal.
Becoming interested in a Rabbi Nachman of Breslov I took to heart a saying he said "It is a big commandment to be happy all the time". So I took another statement of a rabbi (the Rambam) "That your outsides (body) can affect your heart", so I decided to get myself to smile all the time. I just concisously worked to keep myself smiling, and it became a habit.
Although someone commented that it requires a fake smile, it is a lot like doing acts of kindness the more you do the kinder a person you pottentialy can become. The happiness that a smile generates in you does become a part of who you are.
NOTE THIS TAKES A LOT OF EFFORT