Learning how to smile. I would spend time in front of the mirror practicing my smile for different situations.
During interviews I would force myself to smile. It helped get rid of some nervousness. When I get stuck on a problem I would smile stupidly and usually the interviewer would help me out.
When I feel like conversations are getting heated I would take a step back and smile, it helps me direct the conversion to a more positive place.
Sometimes when stuff happens outside my control and life just sucks I'll force myself to smile and handle things to the best of my ability
I also practiced smiling to change my neutral expression (which used to cause people to worry that I might be sad) and now I constantly have a slight smile.
I still haven't gotten used to strangers on the street suddenly smiling brightly at me, it always takes me a moment to realize it was me who smiled at them first.
The secondary effect of receiving unexpected smiles definitely also helps to improve my mood.
Paul Eckman's claim-to-fame is his work of proving that chnaging facial muscles changes the underlying emotions (in extension to the commonly known fact that our facial muscles change to reflect our emotions).
During interviews I would force myself to smile. It helped get rid of some nervousness. When I get stuck on a problem I would smile stupidly and usually the interviewer would help me out.
When I feel like conversations are getting heated I would take a step back and smile, it helps me direct the conversion to a more positive place.
Sometimes when stuff happens outside my control and life just sucks I'll force myself to smile and handle things to the best of my ability