I used to work in a store in a mall. It was hard some days seeing all the sad or angry people walking through so I created a "game" in my head that I call Ambush Smile. Whenever I walked through the halls I look for the most unfriendly or unhappy person I can find. After making eye contact, I smile as warmly as possible. Inevitably, they smile back. Often I would notice them again later still smiling. While retail is no longer my job, I still try to do this whenever I am out and about.