Do hard things. My work -- web dev and programming -- is often challenging. And I have challenging hobbies as well. That's one of the reasons I write fanfic, in fact. Writing is hard. Even though I complain about it being difficult, that's what makes it satisfying.
Mellow out. If you remember your Hitchhiker's trilogy, you'll recall Judiciary Pag (aka Zippo Bebrock 5 x 10^8), LIVR. Learned, Impartial, and Very Relaxed. That has always seemed to me to be the ultimate goal. I will never be laid-back, but I have mellowed considerably in the last five or six years. People and situations just don't irritate me the way they used to. I think a lot of that has got to do with all the reading I do about the brain and consciousness, etc. It's given me a better understanding of why people behave the way they do. And I feel so much better.
Take time. About a month and a half ago, I started working four-day weeks. It's made such a difference. If you can afford to cut your hours, then I would strongly recommend it. I have more time to relax, to work on projects, to have coffee with people.
Make space. I have claimed the spare room as my own. I don't sleep here, but my things are here, my computers, my books, and most of all, my comfy chair. Just having my own area has made me more productive and more relaxed. This recliner was the best thing I've bought in years.
Breathe and smile. Changing your facial expression can affect your mood. If I feel irritated, I try to breathe and smile and often that will help a lot. Whenever I have to type my website password at work, I breathe and smile. I don't mean you have to be a merry sunshine, but this really helps me.
Learn things. I learn best by reading, so I read a lot of non-fiction about the brain, about culture, thought, quantum physics, psychology, writing, anthropology, anything really. I'm hoping to make more time soon for more language learning. And I want to learn to draw, because I royally suck at it and I think studying drawing will make a different part of my brain turn on.
Listen to ABBA. (Take a chance on me.)
What do you do to work at being happy?