Just smile. I’m not sure why smiling feels like a big effort sometimes, but it’s worth trying. Just by having your mouth muscles in a smile, your heart rate lowers and the level of stress hormones in your body goes down. It doesn’t even matter if the smile is genuine or not. You really can fake it ’til you make it. If you use your fingers to draw the outer edges of your lips upward, it still works.
I practice smiling at people. I need to, since it’s not always my first reaction. I suffer from RBF according to my daughter (resting bitch face). When I smile, most people smile back. It makes me feel good, it makes them feel good, and it feels like a small way to pay it forward. You could even build in a habit of smiling at people everyday on the street, in the store, at work, anywhere! Just beware that the opposite also works. Frowning can actually make you feel worse. So, smile more AND frown less. I will too.