Here's what experience has taught me on how I can banish a bad mood fast.
Pump up the music. No don't put on ambient music yet, I find it much more soothing to cue up heavy rock songs on my playlist for a few minutes first just so I could channel my bad vibes to the drum beats and blaring guitar solos. Then when the bad feeling has abated somewhat, I shift to chill and downtempo music. This calms me down and eventually clears my head.
Take a breath of fresh air.
This doesn't just work in the movies, I've only done this a handful of times myself but it really does work. It will leave you alone grueling with your thoughts but the change in scenery will calm you down.
Get yourself distracted. I've read somewhere that you should address bad feelings immediately so that it won't fester deep inside you but what do you do if you really can't do much about it at a given moment? What works for me is that I do something else that would keep my mind off it for a while. Just until I can think straight enough to figure out a solution for (if there is) or source of my bad mood. I'd watch a funny television show or movie for a while or even get out my PSP and play for a bit.
Write about it. I find this the most efficient of all. If you've ever kept a journal then you can relate to what I mean. My journal is where I keep my most inner and private thoughts. This is where everything is uncensored and if I am caught in a really bad mood, I find that writing about it helps ease it in some way. Sometimes after writing about it and reading what I have just wrote, it helps me identify if my bad mood was just a silly reaction.
Vent it out to a friend. If you're not a journal writer with no plans on having one, talking it over with a friend does the same thing. You may just have a friend listen while you rant, there doesn't necessarily have to be a need for him/her to give you advice, just having a friend there to listen helps a lot.
Now the tips I've mentioned above may or may not be the same for you but they work for me. Perhaps you may have other ways that can uplift a bad mood. And in the next time that I have one, maybe I would also find it useful. It would be great if you can share them with me and my readers.
(photos from Flickr creative commons)