THE ACHIEVE IT BLOG has a list of ten ways to improve your mood. The one I liked best was:
"List 20 things you are thankful for...You can’t be genuinely thankful and grumpy at the same time. Try it and see!"
If you've never done this, I think you'll be surprised at how well it works. And it really has to be done by writing. Try to do it in your head and your mind will probably wander, and whenever the mind wanders, it tends to drift into something negative and then it sticks there, bringing you down. So get paper and pen and spend a few minutes writing down what you're thankful for.
Researchers have been trying to find out what makes people happy, and this exercise has been one of the most effective and easiest for people to do. In one study, spending a mere five minutes writing down what they're grateful for every day made people measurably happier.
And you don't have to wait for a reward. You don't have to do it every day as some sort of burden. You can do it today and you will feel better...today.
Why does it work so well? Because the human brain has a negative bias. Your brain is better at noticing what's wrong than what's right. It pays more attention to what's wrong, and thinks about it longer. This may be a good strategy for survival in dangerous times, but doesn't help us feel great.
By deliberately trying to think of what you're glad about, you change the focus of your attention to the good stuff you've been overlooking. It works like magic on your mood.
Further reading: One of my favorite books on the research on happiness and how you can apply it is Martin Seligman's Authentic Happiness: Using the New Positive Psychology to Realize Your Potential for Lasting Fulfillment