Sometimes it helps to notice your physiology if your mood swings in a direction you don't particularly enjoy. I used to fly off the handle a lot until I began to stop and notice what was happening: my body would tense up everywhere. I had to relax first, then exhale. Then I'd hold a smile for ten seconds or more and the negative emotion would be gone. Granted, the first few times I did this, I felt like a silly ape, but it seems to work. Also, putting a more positive context on the situation helps.
If that's not enough, I do silly gestures accompanied with auditory stimulation: air guitar, throwing up my arms and jumping in the air yelling out "America! F*CK YEAH!!!", balling up one hand and doing the "yes!" gesture, or mimicking martial arts moves and pretending to be Neo from the Matrix. Of course, not everyone is so animated and eccentric, so I hope smiling for ten seconds and changing the way you think helps.
Thinking can build momentum. If you start small and make yourself laugh, your mind automatically starts digging up other similar things. What's good about right now? (I use "awesome", but that's a personal preference.) When was the last funny event in your life? How many sexual references are in Aladdin?
A sense of humour helps a lot. And oh, do I mean a lot.
-A
