Making an effort to spread happiness wherever you go won't just benefit those around you, but will benefit you as well. At the core of spreading happiness is serving others. Our society has become very "me-first," so it takes some effort to change that thought process. But the benefits you reap will make it worth the effort.
Volunteer your time. Monetary donations are always appreciated, but most charitable organizations are also in need of people who are willing to give up some of their free time to help their cause. You'll spread happiness to the people you are helping, and spending time outside of your normal surroundings will probably serve to make you better appreciate what you have.
Perform spontaneous acts of kindness for strangers. Something as simple as holding the door for the person coming in behind you, letting someone into traffic ahead of you or letting someone with just a few items in front of you in the grocery line serves to spread happiness.
Smile and be pleasant to those you encounter. You might be having a rotten day, but there's nothing to be gained by spreading it to everyone you meet. Be kind and considerate to the clerk at the store or to the waiter at the restaurant, and you might find that your own troubles seem a little easier to bear.
Repay random acts of kindness bestowed upon you by doing something nice for someone else. By doing this, you allow the happiness to spread throughout your community, making it an overall better place to live.