The answer to this question depends on the field or context from which the respondent is answering. Given that breadth and depth, I'll provide several possible responses and links to more resources you could use in your own response:
- Maintain a balanced diet: Many health professionals believe that the food humans consume has a significant impact on one's ability to cultivate positive emotions. The healthier one's diet is, the happier one feels. This is due in part to the amount of energy one intakes, and to the amount of lipids one's body needs to process.
- Regularly meditate: Meditation, or the practice of conscious focus, is touted by many different religions and medical fields for its positive physical and mental effects, which can strongly influence one's level of happiness.
You might also find some of Gretchen Rubin's book The Happiness Project useful in your research.