How to Scrap a Bad Mood

Find the cause.
Getting right the the source will help shift the heavy cloud hanging over your head. It may take some time, especially if this attitude has been brewing over some time, but finding that connection ends in positive results. The reason can vary, from a person to an experience - figure out what's really bothering you and get as far away from it as possible.
Treat yourself.
To me, any food is a treat. There's absolutely nothing wrong with eating chocolate in every form until the endorphins kick in the lift the sorrow. Indulging in a whole series on Netflix or gobbling multiple tubs of Ben & Jerry's is good for the mental health system and a great start and shifting the negative thoughts. I've also found that herbal tea really comforts my rough spells.
Music.
I have never come across another human being that doesn't enjoy listening to music. It's unnatural not to be drawn to at least one genre or album - there are so many! Finding a pre-made playlist on Spotify or 8tracks is an easy way for me to get out of a poor mood. Avoiding the slow mixed made for break-ups and down days will only aid your mind into plunging into an ever deeper sad state.
Phone a friend.
Socialization will help to lift spirits. Talk out your feelings with someone you really trust and get everything off your chest. Or rather, entertain yourself with a gossip session and catch-up chat on anything and everything going on in the world.
Make yourself laugh.
I'm usually doing this by falling up the stairs on running into a wall. But there are plenty of site, from BuzzFeed to Tumblr that have pages dedicated to bringing a smile. Spending some quality time flicking through Vines is a great way to lift the crummy spirits and create a lighthearted atmosphere.
Get busy!
Distraction is key. An empty mind can quickly fill with lousy thoughts and over-thinking which only increases a bad mood. Find a hobby, whether artistic or academic to really take your mind away from whatever is bringing you down. Throw all your energy into a time-consuming task and the hours will fly by.