Sharing it.
True happiness is shared happiness.
You can be as content as a doorknob, but to be truly content, that contentedness has to be shared with someone.
In terms of return on investment, investing in your relationships has to be up the top of the list for this reason alone.
Make an extra effort to maintain your relationships. Make an extra effort to spend time with the people you care about. When it comes time to feel happy, they all but multiply your happiness.
* this was inspired by a comment shared at an event I was at today