There are nine items to be executed in devotional service. These are enumerated in the following verse from Srimad-Bhagavatam (7.5.23):

Hearing the glories of the Lord, chanting, remembering, serving the lotus feet of the Lord, offering worship in the temple, offering prayers, becoming a servant of the Lord, becoming the Lord's friend, and sarvatma-nivedana, offering oneself fully at the lotus feet of the Lord-these are the nine devotional processes. In The Nectar of Devotion, When Sarvabhauma Bhattacarya asked the Lord which item was most important, Sri Caitanya Mahaprabhu immediately answered that the most important item is the chanting of the holy names of the Lord-Hare Krsna, Hare Krsna, Krsna Krsna, Hare Hare Hare Rama, Hare Rama, Rama Rama, Hare Hare. He then quoted the following verse from the Brhan-naradiya Purana (Thirty-eighth Chapter, verse 126) to confirm His statement.