Happiness is very difficult to define, because it means so many different things to different people. While some people link happiness to wealth and material success, others think it lies in emotions and loving personal relationships. Yet others think that spiritual paths, rather than either the material world or relationships with people, are the only way to true happiness.
Because people interpret happiness for themselves in so many different ways, it is difficult to give any definition that is true for everyone. However, if there are different kinds of happiness for different individuals then the first step in achieving it would be to have a degree of self-knowledge. A person needs to know who he or she is before being able to know what it is that makes him or her happy.
Of course, factors such as loving relationships, good health, the skills to earn a living and a peaceful environment all contribute to our happiness too. But this does not mean that people without these conditions cannot be happy.
Overall, I think an ability to keep clear perspectives in life is a more essential factor in achieving happiness. By that I mean an ability to have a clear sense of what is important in our lives (the welfare of our families, the quality of our relationships, making other people happy, etc.) and what is not (a problem at work, getting annoyed about trivial things, etc.)
Like self-awareness, this is also very difficult to achieve, but I think these are the two factors that may be the most important for achieving happiness.