Contentment (santosha) is one of the five niyamas in yoga. How does one actively practice contentment? Is it through meditation on certain feelings or something else? The other niyamas (cleanliness, austerity, self-study, prayer) seem more obvious in terms of what actions to do. Contentment almost seems like the opposite of actions. With contentment, what everyday actions does one take to practice this part of yoga?
As answer to your question, below is an excerpt from Yoga-Vasistha (Discourse of sage Vasistha to Prince Rama to explain the illusory nature of the manifest world and the principle of non-duality). The sage told about four gate-keepers to the realm of freedom. The four gate-keepers are Santi (self-control or quietness of mind), Vichara (spirit of inquiry), Santosha (contentment) and Satsanga (good company). Here (search page 6) are the words of Sri Vasistha about Santosha (contentment):
What is contentment? To renounce all craving for what is not obtained unsought and to be satisfied with what comes unsought, without being elated or depressed even by them — this is contentment. As long as one is not satisfied in the self, he will be subjected to sorrow. With the rise of contentment the purity of one’s heart blooms. The contented man who possesses nothing owns the world.
Therefore, it is renunciation of powerful desire on one hand and satisfaction with what we have on the other that forms the practice of contentment.