I agree with the general sentiment in this thread that happiness is a personalized concept, that can only really be built by the individual in question. However, I think there are some generally agreed upon rules or ideas that help avoid unhappiness. Perhaps I can take a stab at listing a few of the main ones that have helped me:
Honesty - this can run deep. If you carry a bunch of lies with you for your entire life, you'll be stressed out and feel the need to make reparations for your lies; the guilt can be crippling. Honesty with yourself also breeds confidence, and mindfulness.
Gratefulness - appreciating the things you do have, right now, instead of coveting the possessions of others (often because you see the happiness it brings them [often because they appreciate what they have]) will do wonders for your overall spirit, sense of love, and sense of self. People with almost no money and possessions often are happier in terms of heart and soul than the richest wall street tycoons.
Vocation/Calling/Passion - I have hated my job for almost two years now. It is palpable how much resentment I carry around with me about the work I do, and how little it means to me, and how much of a drain it is on my physical and emotional health. The days I put something into it, despite all of this, I feel like I can be proud of what I did, if even just a small amount in between the doldrums and home. This is not an argument for happiness equating to finding your "true" calling (for some, their calling is actually the act of trying to find their true calling, however it argues that working hard is good for the soul in some ways. It releases you from guilt, it's inclusive rather than exclusive.
Communication Skills - This is a bit less over-reaching than the rest of my list, but as I grow up and enter further into the adult world, I am learning that this is incredibly important. I say this because I believe that life is a series of experiences that are strung together with relationships between people, and between a person and his/her "stuff" (emotional content, passions, hobbies, animals, surroundings). When you don't communicate, things build up and become out of control. I'm finding it even rather difficult to write about - staying on top of communication in your life, making it simple, straightforward, and confident is so important. Not easy, of course. But, its amazing the power of being upfront with people about things that you feel, or affect you. Its an exercise in self-understanding, almost. The more you talk about how the world around you affects you, the more you can understand the inevitable changes in your life and roll with them, even get something out of them.
Hope this helps! Mindfulness is a HUGE part of what I truly believe to be the path to happiness in this temporal plane (and after, really).